Two separate registries that cover the whole population of Finland have been linked in order to study the occurrence of cancer in patients with rheumatoid arthritis. The two registries involved were the Social Insurance Institution's Population Register, which collects information on medication for certain chronic diseases since 1967 including rheumatoid arthritis, and the Finnish Cancer Registry, which has data on all cancer patients in Finland since 1953. The 213,911 person-years' follow-up of 46,101 individuals with rheumatoid arthritis revealed 1,202 cases of cancer. The relative risk of cancer was 1.15 in males and 1.01 in females. The observed number of cases significantly exceeded that expected in Hodgkin's disease, non-Hodgkin's lymphomas, multiple myeloma, and leukemia. In general, the incidence of epithelial tumors was as expected. These findings were further supported by studying the occurrence of subsequent new primary cancers among patients with rheumatoid arthritis and cancer.