During January to July 1986, 1,500 fecal specimens from out and in-patients attending Siriraj Hospital, Bangkok were examined for Cryptosporidium oocysts by modified acid fast technique. Eight were found to be positive. The prevalence of cryptosporidiosis was 3.7% in children with acute diarrhoea. Medical records of the patients showed that the main clinical presentation was acute diarrhoea with fever for one to 7 days duration. Stool examinations and cultures revealed no white blood cells or parasites or other enteropathogenic bacteria. All recovered within two weeks. The study suggest that Cryptosporidium is a relatively common nonviral cause of acute diarrhoea in young children and routine laboratory study to detect Cryptosporidium may be justified.