The incidence of cancer was evaluated in a population-based cohort of 959 patients diagnosed with seizure disorders while residents of Rochester, MN, between 1935 and 1979. For all cancer sites combined, there were 65 incidence cases for a standard morbidity ratio of 1.4. Most of the excess was attributable to a 22-fold increase in the incidence of primary brain tumors among these patients. Most of the brain tumors occurred within 5 years of the seizure disorder diagnosis, suggesting that the seizure disorder was due to the brain tumor. The incidence of cancer of other sites, exclusive of brain, was not elevated. There was no evidence of an association of cancer incidence with duration of seizures or with the use of anticonvulsant drugs.