We have studied the clinicoetiological, serological, and molecular epidemiological features of adenoviral conjunctivitis under the auspices of the nationwide surveillance project in Sapporo, located in northern Japan. We were able to analyze the results of 1,454 cases of adenoviral, herpetic, and chlamydial conjunctivitis. We found that adenonovirus serotypes 8, 19, and 37 caused severe conjunctivitis without systemic symptoms, whereas serotypes Ad3, Ad7, and Ad11 caused mild conjunctivitis with systemic involvement. Ad4 showed a broad range of symptoms, from PCF to EKC. Adenoviral conjunctivitis had seasonal outbreaks in the summer in association with adequate temperature and humidity in Sapporo. Neutralization antibodies against Ad8, Ad19, and Ad37 were detected in fewer than 20% of cases, and so the incidence of epidemics with these serotypes may rise in the coming years. The main genome types of adenovirus in Sapporo were Ad4a, Ad8e, Ad19a, Ad19b, and Ad37p, a, and b.