OBJECTIVE The objective of this study was to evaluate the performance and effectiveness of a register of employees exposed to carcinogens (the ASA Register) which has been in operation in Finland since 1979, and to study cancer risks among the notified workers. METHODS The impact of ASA at workplaces was studied by questionnaires mailed to 1448 work departments, which were notified to ASA in 1996, and to 1033 departments, which departed ASA in 1991-1996. The mailing was responded by 69% of departments. The cancer incidence of 35,138 workers notified to ASA in 1979-1988 was followed up through the files of the Finnish Cancer Register for the period 1980-2003. RESULTS Changes eliminating or substantially reducing exposure to carcinogens were reported by 73% of departments notified to ASA in 1996. The ASA notification process had directly prompted measures to reduce exposure (8% of cases) or contributed to them (24% of cases). Estimations based on responses of the workplaces suggested that the ASA registration had decreased exposure of 600 workers year(-1) (out of approximately 15,000 notified workers, which is <1% of the employed in Finland), preventing thereby an unknown number of occupational cancers. Other benefits of ASA included the saving of the treatment costs of prevented cancers, the prevention of other health outcomes of carcinogens, improved safety behaviour of exposed workers and avoidance of human suffering among cancer patients and their families. The labour safety authorities had better possibilities to direct their activities against carcinogen exposure. These benefits should be considered against the annual costs, mainly due to 7-8 person-years of work required by tasks related to ASA. The results of the cancer incidence study among notified workers were based on a relatively short follow-up (on average 19 years). The incidence of mesothelioma was significantly increased in the ASA cohort, probably due to exposure to asbestos. CONCLUSIONS These results suggest that a national exposure register may stimulate preventive measures at workplaces. Partially based on the results of the present study the Finnish Ministry of Social Affairs and Health continues ASA registration.