Human astrovirus was detected during a 13-month longitudinal study of the incidence of diarrhoea viruses among hospitalized children (less than 2 years of age) in a pediatric clinic of the city of S. Paulo (University Hospital). Serial fecal samples (intervals of 4 days) were collected from 146 children with and without acute diarrhoea at admission and during their stay in the hospital. Two (3%) of the 67 children with diarrhoea were positive at admission to the clinic by the highly sensitive ASTROVIRUS BIOTIN-AVIDIN ELISA, using CDC monoclonal antibodies (MAb SE7). All 79 children without diarrhoea (controls) were negative for astrovirus at admission. However, astrovirus was detected in 7 (4.8%) of the 146 hospitalized children during their stay in the clinic. Three of the positives shed astroviruses in 2 successive stool samples. All children positive for astrovirus were negative for rotavirus, adenovirus and bacterial and parasitic enteric agents. This is the first study of the detection of human astrovirus in Brazil. Astrovirus appears to be a significant cause of infantile gastroenteritis among young children in this country.