From April 16 1987 through May 16 1987, during an outbreak of gastroenteritis, stool specimens were obtained from 53 children aged 18 to 36 months among the 90 children attending an on-site day-care center for the staff of a large teaching hospital in the Paris urban area (59%). Oocysts of Cryptosporidium were found in 11 specimens (21%) using an auramine staining technique. Children with diarrhea were more likely to have stools containing Cryptosporidium (p less than 0.01). Subsequently, a prospective study was carried out in the same day care center from July 1987 through January 1988. Among the 103 episodes of diarrhea observed during the study period, there were five cases of cryptosporidiosis (5%). In all these cases, diarrhea was moderate and resolved within ten days. Furthermore, among 148 hospitalized children aged 2 months to 10 years, 2 (1.4%) had positive stool specimens for Cryptosporidium and significant failure to thrive. Thus, Cryptosporidium is a common cause of diarrhea in immunocompetent children, especially in child group settings. Further studies are needed to determine the prevalence and spectrum of the clinical patterns of this parasitic disease.