The aetiology of rotavirus and adenovirus in acute gastroenteritis was studied in a prospective series that comprised 283 children admitted consecutively with diarrhoea during a 1-year period. Rotavirus was associated in 49% of the cases by solid-phase radioimmunoassay and electron microscopical examination of stool specimens, or by serology. Adenovirus was detected by radioimmunoassay in the stool specimens of 29 (11%) patients, including 8 cases of possible dual infection with rotavirus. Rotavirus infections showed a typical age distribution and seasonal clustering between January and June, whereas the adenovirus-associated cases did not form a distinctive subgroup. Enteropathogenic bacteria were found in 10% of cases, and were nearly as common in association with rotavirus infection as not. Respiratory symptoms accompanied diarrhoea in 34% of the patients with rotavirus and in 25% of those with neither rotavirus nor adenovirus. Therefore we could not confirm the existence of a 'rotavirus syndrome', nor could we confirm an association of respiratory symptoms with rotavirus infection. Use of antibiotics before the onset of diarrhoea was more common among those with non-viral diarrhoea (23%) than in the rotavirus group (13%). Rotavirus infections appeared to be common among cases of 'antibiotic-induced' diarrhoea.