A multisource medical register review identified persons with active epilepsy in northern Sweden. Seven hundred thirteen persons aged greater than or equal to 17 years with epilepsy were determined on the prevalence day, December 31, 1985. The overall prevalence was 553 in 100,000 (566 in 100,000 if adjusted to the 1980 U.S. population). The ratio of males to females was 1.1, with a male prevalence of 575 and a female prevalence of 530 in 100,000. Age-specific prevalences varied between 530-644 in 100,000 except in persons aged greater than or equal to 70 years, for whom the prevalence was 321 in 100,000. Partial seizures were most common, 333 in 100,000, of whom the majority (250 in 100,000) had seizures that occasionally were secondarily generalized. Mental retardation was the foremost coexistent disorder, noted in 23%. The mean yearly seizure frequency was higher in persons with mental retardation than in nonretarded persons. Seventeen percent had seizures during the last week, 57% during the last year, whereas 16% had greater than or equal to 5 years' freedom from seizures. Most had onset of epilepsy before age 20 years. A presumed etiology was noted in 35%, more often in men than in women. Cerebrovascular disease was the most commonly identified presumed cause. Other nonepileptic diseases/disabilities were noted in 47%.