I came to work for the Cancer Registry in the summer of 1975 to develop the computer system. That “brief employment” has now stretched to 47 years. Alongside my computer system responsibilities, my role soon evolved into research work based on data collected in the registers. The Cancer Registry already had world-class researchers in charge of secondary and tertiary cancer prevention studies: Matti Hakama, a mass-screening guru looking for ways to find cancers as early as possible, and Timo Hakulinen, a master of survival analysis identifying factors that influence cancer survival. But there was room on the primary prevention side, so I set out to find factors that increase the risk of cancer that could be eliminated to prevent cancers from occurring. There was a growing call in the 1970s for efforts to reduce the main cause of cancer, smoking. At the initiative of the National Board of Health, the Cancer Registry calculated the extent to which different smoking reduction strategies would prevent cancers. Would it be to entice habitual smokers to kick the habit or to target resources at young people to reduce the number of new smokers? In the first option, the effects would be immediate, while in the second, the benefits would be felt after more than 20 years but would be greater. Together with Timo Hakulinen, we made the desired projections up to 2050. We now know that the annual number of men with lung cancer is less than half of what was considered possible in the most over-optimistic estimate by the Finnish Medical Council. The prevalence of risk factors can therefore be influenced. The Cancer Registry was tasked with “monitoring populations at potentially increased risk of cancer”, which was used as an extension of the cancer database to collect files on people deemed necessary to be monitored. These included people who worked in working conditions that were feared to be associated with cancer risk or who lived in areas perceived to be at risk of cancer. In this paper, I describe the investigation of cancer risk associated with living and working conditions. The Finnish Institute for Occupational Health compiled a statutory ASA register of people exposed to carcinogens in their work environment, and the Cancer Registry played a key role in its design and subsequent cancer surveillance. Workers in the rubber, glass, oil refining, metal and chemical industries, among others, were included in the survey. Cancer surveillance data should therefore be followed for very long periods of time. In many studies, it has happened that a publication is made of recent data, with the final review stating that the review period is too short to draw firm conclusions and that a new review is needed at some point in the future. Data on all persons who worked in asbestos quarries in Finland had already been compiled in the 1960s under Lauri Meurman and Matti Hakama, but there was still new information on their cancer risks in the 2020s. This is a good reminder that the time from exposure to the development of cancer is usually long, decades in the case of asbestos. There were several hundred records in the Cancer Registry, kept up to date by both the ever-changing data processing technology and the annual death updates, so that at any given time, if necessary, cancer risk figures could be calculated with about a day’s lead time. Unfortunately, under current research authorisation interpretations and practices, it has become almost impossible, if not illegal, to keep the data in the Cancer Registry for future research. The Cancer Registry persevered for years in an unsuccessful attempt to gain archiving rights to research data, and in the 2020s a large number of data finally had to be destroyed before all the information on the cancer risk of the populations concerned had been clarified. One of the most publicised of these datasets in the world was that on flight attendants, which was originally compiled because it was suspected a cause of cancer among them was due to passenger smoking in airplanes. This conjecture did not materialise, but then the study turned its attention to the dangers of cosmic ionising radiation. In 1995, the world’s first publication on this subject by the Cancer Registry showed that women who had worked as flight attendants for a long time had twice the incidence of breast cancer compared to the rest of the female population. The discovery triggered a series of multinational research projects on the cancer risk of flight attendants, and soon the research was extended to include pilots. The breast cancer finding was repeated everywhere, but no good explanation was found. Theories suggested that such a small amount of radiation could not have caused such a high risk. In the 2000s, it was suggested that circadian rhythms might be the cause of cancer. Studies of flight crews were an important justification for the IARC’s classification of night work as a likely risk factor for cancer. Because the findings on flight crews could be generalised to include frequent flier business people, this Finnish study made the cover of Time magazine. The work of compiling and reprocessing separate workplace data into a high-quality survey data set was such that in the 1990s the Cancer Registry began to consider studying occupational cancer risks using information already available from the registers. The censuses asked each person’s occupation every five years, and the data were coded and stored at Statistics Finland. When this information was combined with data from the Cancer Registry, the risk of any cancer in any occupation could be calculated “at the click of a button”. In addition, the Finnish Information System on Occupational Exposure (FINJEM) matrix, which showed how much exposure to chemicals, dust, stress, lack of exercise and other factors that might affect cancer risk at any given time in any given occupation, and the prevalence of smoking, alcohol consumption and obesity in different occupations, was compiled by a team of experts led by Timo Kauppinen of the Finnish Institute of Occupational Health and provided a tool for estimating the impact of occupational risk factor doses on cancer risk. The work started in Finland, but with the Nordic Occupational Cancer Study (NOCCA) it was extended to all the Nordic countries, and the same data and methodology were eventually used by researchers in dozens of countries. The efficiency of the Finnish registry-based research line, which is at the international top level, is demonstrated by the fact that in the best years the list of publications of the Cancer Registry was extended by a new publication on average once a week. The importance of the research is demonstrated by the fact that my primary prevention publications alone have been cited some 60,000 times in scientific publications around the world.