• Cryptosporidium was found in the stools of 13.5% of 221 children hospitalized with diarrhea. It was the single most prevalent pathogen isolated. Children with Cryptosporidium-positive stools were significantly more malnourished than children in whom Cryptosporidium was not detected. Children with more severe malnutrition (ie, less than 50% of their expected weight) and with Cryptosporidium in their stools had a significantly longer duration of diarrhea than similarly malnourished children without Cryptosporidium (63 vs 32 days, respectively). In 77 better-nourished outpatients with diarrhea, Cryptosporidium was found in only 5.2% of cases and was associated with lesssevere illness. Our findings are consistent with the hypothesis that in less-developed areas, Cryptosporidium is a major pathogen, not only in acute but also in chronic childhood diarrhea, and may play an important role in the interaction between diarrhea and malnutrition.