We conducted a 1-year prospective study in two clinics and a hospital in the Negev region of southern Israel to determine the epidemiological and clinical patterns of Cryptosporidium diarrhea among Jewish and Bedouin infants and children living in the same geographical area. A total of 612 episodes were studied: 398 in Bedouins and 214 in Jews, of which 449 occurred in patients with diarrhea and 164 in controls. Cryptosporidium was detected in 13 of 382 patients (3.4%) with diarrhea and in 1 of 138 controls (0.7%) (P = 0.078). In 5 of 13 Cryptosporidium-positive patients (38%) another pathogen was detected. No significant difference in Cryptosporidium detection rates was observed between Jews and Bedouins or between hospitalized or nonhospitalized patients. The frequency of Cryptosporidium detection did not differ significantly when three age-groups were compared (less than 6 months old, 7-12 and 13-36 months old). The rate of Cryptosporidium detection was similar among malnourished and well-nourished patients, as determined by weight-for-height percentiles. Cryptosporidium was detected more frequently during the summer months (8.3%) than during the rest of the year (1.2%) (P less than 0.001). Patients with Cryptosporidium diarrhea did not differ clinically from patients with other causes of diarrhea. However, they were characterized by the absence of fecal leukocytes. Cryptosporidium is not a rare cause of diarrhea in southern Israel. It is more prevalent during the hot and dry season and can be detected in a relatively high prevalence among very young infants. Its clinical features are indistinguishable from those of patients with non-cryptosporidial diarrhea.