U.S. polio vaccines produced during the 1950s were potentially contaminated by simian virus 40 (SV40). Recently DNA from SV40 has been detected in brain ependymoma, pleural mesothelioma and osteosarcoma. In 1957, when national polio vaccination was started in Sweden, vaccine potentially contaminated with SV40 was given to approximately 700,000 individuals, mainly pre-school and school children born between 1946 and 1953. From 1958, a Swedish inactivated polio vaccine was exclusively used, which has been claimed to be free of SV40. We explored cancer incidence rates in the cohorts exposed to the potentially contaminated polio vaccines in Sweden. The Swedish Cancer Registry provided annual cancer incidence rates in five-year age groups for the years 1960-93. Cancer incidence in cohorts maximally exposed was followed during this period, and the incidence when these cohorts reached a specific age was compared to the incidence when unexposed cohorts reached the same age. For osteosarcoma and brain ependymoma overall age-standardised incidence rates were essentially unchanged between 1960 and 1993, and age specific rates were similar in the exposed and unexposed male and female cohorts. During the same period, overall age standardised incidence rates in males of brain cancers increased from 9.0 to 13.1 and of pleural mesotheliomas from 0.2 to 2.1 per 100,000. None of these increased rates was associated with the exposed cohorts. The use of potentially SV40 contaminated inactivated polio vaccines in Sweden has not been shown to be associated with increased cancer incidence. However, the exposed cohorts have not yet reached the age of increased risk of brain cancer or mesothelioma.