The incidence of idiopathic/cryptogenic epilepsy and isolated unprovoked seizures has been relatively stable in the population of Rochester, Minnesota, for the 50-year period 1935 through 1984. In each decade, the age-specific rates exhibited a consistent U-shaped pattern of decreasing rates from infancy to age 40-49 and a progressive increase thereafter to a second peak at age 70 years. Males had a 15% higher incidence of cryptogenic unprovoked seizures than females. The most pronounced secular trend was a decrease in the incidence in children aged < 10 years for the first 4 decades of the study; however, this trend was interrupted by a slight rebound in the decade 1975-1984. There has been a progressive decrease in the incidence of cryptogenic unprovoked seizures in individuals aged > or = 50 from 1965 through 1984. This decrease paralleled the decrease in cerebrovascular disease in the community. The decrease in idiopathic unprovoked seizures may be related to a concurrent trend in "silent stroke." Plots and Poisson regression analysis did not show patterns in the incidence of idiopathic unprovoked seizures related to successive birth cohorts. However, significantly lower incidence rates were observed for the 1930-1934 birth cohort, about half that of all others, between the ages of 5 and 54 years.