Cryptosporidiosis is an intestinal disease caused by the protozoan Cryptosporidium parvum. The most common manifestation in human is enteric symptoms, which in immunocompetent hosts are self-limiting but can be life threatening in immunocompromised hosts, characterized by profuse watery diarrhea, abdominal pain, severe weight loss. It's prevalence rate in immunocompetent host is variable by geographic locations (3~11%) but up to 15~40% in AIDS patients. Now it is considered as one of the important enteropathogens causing diarrhea not only in immunocompromised but also in immunocompetent hosts, especially in children. We experienced two cases of cryptosporidiosis in a 15 year old boy who was admitted due to diarrhea , abdominal pain and fever and in a 8 year old boy who was admitted due to watery diarrhea and vomiting. These are the first clinical cases of cryptosporidiosis confirmed by electron microscopy of the colonic mucosa among immunocompetent children in Korea.