Because of increasing numbers of incident cancers and improving survival rates, more and more cancer patients are at risk of subsequent primary malignant neoplasms. The potential carcinogenicity of cancer therapies raises further concern regarding this unwanted long-term side effect. The objective of this cancer-registry-based study was to assess the trends in the risk of subsequent primary malignancies among cancer patients on a nationwide public-health level. The 470,000 cancer patients registered between 1953 and 1991 in Finland were followed up for subsequent primary neoplasms, and 19,800 subsequent malignant neoplasms were observed, yielding an overall standardized incidence ratio (SIR) of 1.00 for men and 1.25 for women. Substantial variation was observed in the SIR by the site of subsequent malignancy. The adjusted relative risk derived from multiple regression analyses showed only a slight increase by follow-up interval. Overall, the excess relative risk of subsequent malignancies was low, since the majority of cancer patients are old and at low relative risk. Patients under the age of 50 years, however, were at 70% increased risk, and the overall relative risk increased by 50% from the 1950s to the 1980s. Subsequent neoplasms among cancer patients do not pose a major public-health problem, but the possibility of subsequent cancers must be borne in mind in decisions concerning therapy and in clinical follow-up.