Twenty-one adult in-patients (11 women, 10 men, aged 18-39 years) with uncontrolled epilepsy participated in a 4-week intensive physical training program, exercising for at least 45 min three times a day, 6 days a week, at an intensity of minimum 60% of maximum oxygen uptake (maximum VO2). The program induced a considerable increase in maximum VO2 (mean 19%). Beneficial psychological and social effects were also recorded. The average seizure frequency during the 4-week exercise period was compared with 2 preexercise and 2 postexercise weeks. There was no significant difference, but there was considerable variation among patients. Only six patients had seizures during exercise. The occurrence had no relation to seizure type, mode of activity, or pulse rate. We conclude that physical activity does not represent an important seizure-inducing factor in general, and that in most people with epilepsy physical training appears to have a favorable influence. The exact mechanism behind this influence is not known, but physiologic as well as psychological and social effects may be of importance. Physical training did not change the serum levels of the antiepileptic drugs to a clinically important degree.