Diarrhea, mostly chronic diarrhea and weight loss are common in patients with AIDS. Cryptosporidium had been identified as responsible for chronic, debilitating secretory diarrhea in HIV infected patients. We performed a retrospective study of the prevalence, clinical features and laboratory findings of cryptosporidiosis in HIV infected patients (adults and children), in the period of 6 years from January 1988 to December 1993 at Bamrasnaradura Hospital in Nonthaburi, Thailand. In this study, Cryptosporidium was found in 22 (8.8%) by detection in stool specimens of 250 HIV infected patients with diarrhea and was found throughout the year. The prevalence rates of cryptosporidiosis in this study among children and adults were 19% and 7.9%, respectively. The common features were chronic diarrhea (84.6%), mostly watery diarrhea and weight loss/malnutrition (100%). A few fecal leukocytes were found in 15.4%.