Background.Human astroviruses cause infantile gastroenteritis worldwide, but the prevalence of disease varies greatly by setting. Since 1997 we have conducted a survey to determine the causes of diarrhea among Spanish children attending an emergency room in Madrid and to characterize the clinical features of viral-associated gastroenteritis.Objectives.To define the epidemiologic role of astrovirus-associated gastroenteritis in Spanish children, to review its clinical features and to compare these illnesses with those caused by rotavirus. To assess the sensitivity of two methods of detection [enzyme-linked immunosorbent assay (EIA) and reverse transcriptase (RT)-PCR].Methods.Fecal specimens from 822 children with acute diarrhea treated at an emergency room were screened by EIA assays. Random astrovirus-positive samples were characterized by RT-PCR and nucleotide sequencing for their phylogenetic grouping.Results.Astrovirus was detected in 44 (5.3%) of 822 specimens tested by EIA. No pathogens were detected in fecal specimens from 238 (29%) children; however, in 137 of those with adequate remaining specimens, we found an additional 50 (6.1%) that were positive by RT-PCR. HAstV-1 was the most prevalent type followed by HAstV-2. The gastroenteritis associated with astrovirus alone was slightly less severe and had a lower score or risk of hospitalization than that associated with rotavirus (P< 0.05).Conclusions.Astrovirus was found in 11.4% of all children whom we tested for enteric viral and bacterial pathogens, making it the second most common cause of acute gastroenteritis among Spanish children. True prevalence of astrovirus could be underestimated if only EIAs were used for detection.