During a 2-yr period starting on 22 July 1979, Campylobacter jejuni was isolated in 3,008 stool cultures from 2,450 persons--2,430 (43.%) out of 56,439 patients with acute gastroenteritis and 20 out of 200 asymptomatic household contacts. No isolates of C. jejuni were obtained from any of a control group of 222 subjects. In 176 patients with C. jejuni, Salmonella and/or Shigella were isolated concomitantly. All illnesses were mild to severe and all patients, including 61 requiring hospitalization, recovered uneventfully. Patients' ages ranged from 2 wk to 88 yr, with infants and children comprising 84% of the total. Ninety-one institutional and 36 family outbreaks accounted for 606 patients. Except in two cases related to C. jejuni-infected dogs, no source or route of transmission was established. Sixty-two percent of the patients came from 18 urban localities and 38% from 125 rural settlements widely dispersed throughout northern Israel.