Abstract: The present study was conducted during the period May 2003 to October 2003 in 616 children less than three years of age with and without diarrheal disease presenting at the pediatric clinic of teaching hospitals in Kermanshah, Iran. Single stool specimens were collected from 515 diarrheic and 99 non-diarrheic children. Two smears were made from each stool samples and were stained by a modified Ziehl Neelsen technique. Cryptosporidium parvum (C. parvum) were detected in 10.4% of children. The C. parvum infection rate was significantly higher in diarrheic children (11.6%) than in non-diarrheic children (4.0%). C. parvum was observed more frequently in stool samples of children who lived in rural areas (15.2%) than those who lived in urban areas (7.2%). In regard to the presence of animals, the infection rate was 18.5% among children who lived in association with animals in comparison with 8.2% among those who lived in compounds with no animals. The majority of C. parvum cases occurred in children between the ages of 0-12 months (11.9%), followed by in children between the ages of 13-24 months (9.2%) and in children between the ages of 25-36 months (3.0%). The data suggest that C. parvum is relatively endemic in young children and that Cryptosporidium may be an important pathogen associated with diarrhea in young children.