Throughout the history of the CSF, cancer screening has been one of its most important activities – at some stages even the most important. It has therefore been natural that the organisation’s own research body, the Cancer Registry, has developed a solid scientific basis for its activities. There are probably hundreds of different candidates for screening tests offered by the basic sciences. Of these, only a few dozen have progressed to clinical use. On a broad scale, three screening tests are used in Finland: cervical cancer, breast cancer and colorectal cancer. Finland has been a global success story in terms of the goal of screening – preventing cancer deaths. The Mass Screening Registry was started in 1968. Even before that, part of the mass screening activities of the CSF had been carried out by the Cancer Registry. The establishment of sub-registries was part of Erkki Saxen’s development strategy. A person in charge and the necessary staff were appointed for the sub-registry, thus making it possible to develop it as a focal point for research in this field. Since the 1960s, cervical cancer screening has been a central part of the work of the CSF. It had set up cytology laboratories in the regional cancer societies and trained cytologists and cytology assistants. Sakari Timonen had introduced cervical cancer cytology to Finland, and Niilo Voipio had harnessed the CSF – and, through it, the health service – to carry out screening in the form of population-based screening. Voipio’s vision and contribution in particular were crucial to the fact that, as screening expanded, it became a form of CSF activity. The Mass Screening Registry was set up to assess the effectiveness of screening. The task required planning that allowed for a scientifically valid study. It was based on a public health approach, i.e. the evaluation was based on population-level mortality (and incidence) changes. The target population was invited to be screened in person, participants and non-responders were registered, and the cancer risk and mortality risk of the different groups was monitored using the Cancer Registry. The screenings done by the CSF thus became a model for an organised screening programme. In modern terms, this would be called a screening chain. Breast cancer was the next screening target. The screening test was a mammography film, which was analysed at provincial screening centres. The Mass Screening Registry was involved from the beginning as an active planner. The main scientific extension was randomisation of invitees and controls. Screening was targeted at women born in a specific year, birth years were extended to new cohorts, until within a few years all women of a certain age were invited to be screened. Screening, like any national reform, would in any case have been a gradual process, with a planned organisation allowing for an evaluation of its effectiveness. In addition to the active contribution of the CSF, the support of National Board of Health officials and a regulatory framework were crucial, without which the impact assessment would not have been realised. The Mass Screening Registry assesses the whole screening chain, not just individual parts of it, such as the accuracy of a laboratory diagnosis or compliance with a referral recommendation. This provided information on how well the screenings were achieving their basic objective of reducing mortality from screenable cancers. The scientific evaluation of the effectiveness of the three cancer screening programmes built into the health care system, cervical cancer, breast cancer and, as the latest entrant, colorectal cancer, has a sophisticated trend: the post-intervention evaluation of cervical cancer screening, the pre-intervention planning of breast cancer screening with group screening and the pre-intervention planning of colorectal cancer screening with individual screening. The theory of randomised healthcare research became one of the priorities of the Mass Screening Registry. Alongside and as a result of the conceptual developments, the statistical formulae for the empirical analysis were derived. In contrast, prostate cancer screening was evaluated as an international epidemiological collaborative study, an (individual) randomised screening trial. The collaboration between the University Hospital of Helsinki (laboratory, Ulf-Håkan Stenman), the University of Tampere (epidemiology, Anssi Auvinen, and urology, Teuvo Tammela) and the international consortium (Erasmus University, Fritz Schröder) was seamless. PSA-based sampling as a screening test was widespread in Finnish health care without any rigorous research on its mortality impact. In general, prostate cancer screening differed from the previous ones in terms of scientific knowledge. The aim of the study on routine screening was to determine whether screening that has been shown to be effective can be applied in Finland with at least an acceptable level of reproducibility. In the case of prostate cancer screening, this knowledge was obtained precisely through this international scientific collaborative study. Practical differences from previous routine screening evaluations included funding from research sources and the ethical rules of the study. In Finland, cancer screening has prevented at least two thirds of cervical cancer, a quarter of breast cancer and a seventh of colorectal cancer deaths. The relative effectiveness of each new screening started has thus been lower than its predecessor, and it seems that the expansion of cancer screening to new types of cancer is slowing down, perhaps even stopping. PSA screening for prostate cancer was able to prevent one in ten prostate cancer deaths, but due to its high harms, screening has not been initiated as part of routine healthcare. Of course, the research work of the Mass Screening Registry covered many issues related to the development of routine screening. For example, the cervical cancer screening test was modernised or replaced by automation technology, which required evidence of either improved sensitivity or a reduction in overdiagnosis compared to the traditional test. The health impact of splitting the CSF’s single screening chain had to be assessed. The replacement of population screening by risk group screening was also frequently advocated. The theory of such targeted screening was one of the priorities of the Mass Screening Registry. The history of the Mass Screening Registry of the Finnish Cancer Registry mirrors the general development process of cancer screening mentioned at the beginning. Laboratory sciences produced a wealth of candidates for screening tests, many of which progressed to clinical practice, but few were eventually found to be effective at the population level. The outcome was ultimately determined by the weakest link in the screening chain, no matter how effective the other links in the chain were. In this respect, too, the Finnish healthcare system has shown its enlightenment: only screening for cervical, breast and colorectal cancer is carried out as population screening Eli as a public health service. The oft-repeated policy prescription or objection to such criticism – “it is better to do something than nothing” – is simply not true. Few proposed cancer screenings are beneficial, but every one has drawbacks.