The present study investigated the prevalence rate of Cryptosporidium parvum as a cause of diarrhea. We examined 942 stools of unidentified reasons occurring in patients in whom no immunosuppression had been detected. We examined the stools for Cryptosporidium parvum via modified acid-fast staining. The clinical records of all of the positive patients were then analyzed. Nine ($1\%$) of the stools among the 942 diarrheal patients were positive for C. parvum. The positive rate in the males was $1.1\%$ (6/522) and the positive rate of the females was $0.7\%$ (3/420). Age distribution revealed that the highest positive rates were in patients in their sixties, with a positive rate of $2.5\%$ (4/158). In the clinical tests, levels of c-reactive protein, erythrocyte sedimentation rates, and neutrophil proportions were normally increased in the peripheral blood, whereas the lymphocyte proportion exhibited a tendency towards decrease. The pathological findings were compatible with an inflammatory reaction in the host.