We report a 5 year prospective study of episodes of rotavirus, subgenus F adenovirus, and astrovirus gastroenteritis diagnosed by electron or immune electron microscopy in a single regional virology laboratory. Of 1426 total infections, the numbers in each category were 1117 (78.3%), 254 (17.8%), and 20 (7.9%), respectively. Using restriction endonuclease analysis or immune electron microscopy, all but 20 of the subgenus F adenovirus strains were classified as type 40 (n = 50) or type 41 (n = 184). Rotavirus and astrovirus infections were more prevalent in winter than summer, whereas subgenus F (either type 40 and 41) adenoviruses showed no seasonal variation in prevalence. The ratio of type 40 to all typable subgenus F adenoviruses declined between 1984 and 1986 and then increased again. Adenoviruses were relatively more important as causes of viral gastroenteritis in infants aged less than 6 months than in toddlers aged 12 months or more, but even in young infants more rotavirus than adenovirus infections were diagnosed. Our data confirmed the epidemiological differences between rotavirus, subgenus F adenovirus and rotavirus gastroenteritis and documented the shared epidemiological characteristics of type 40 or 41 adenovirus infections.