This report summarizes both a prospective study of diarrhea in cohorts of rural children in their natural ecosystem, and a vertical study of diarrheic urban children attending a hospital emergency service. Cryptosporidium oocysts were found in feces of 4.3% of the cases, while all controls were negative. No infection occurred in the first year of life among rural infants, contrasting with a 3% infection rate in children under 1 year of age in the metropolitan area. This could be attributed to intense and exclusive breast-feeding for several months in the rural area while in the urban area many infants are not breast-fed at all, or are weaned prematurely. No infection was found in wholly breast-fed infants. Diarrhea associated with Cryptosporidium was watery and without inflammatory cells. Dehydration was common in urban children, but was rapidly corrected by oral rehydration therapy, or by intravenous fluid therapy in some cases. Infections clustered in the warmer, rainy and humid months of the year.