In four crowded townships of Lusaka, Zambia, the prevalence of cryptosporidiosis in 222 children with diarrhea was 18%, with marked temporal and geographic variation over the course of one rainy season. Using data on the finding of oocysts of Cryptosporidium parvum in urban water supplies, the areas under study were categorized as high or low risk. Prevalence of cryptosporidiosis in children with diarrhea was higher in high risk areas after stratification by early/late stage of the rains (Mantel-Haenszel odds ratio [OR] = 2.9, 95% confidence interval [CI] = 1.3, 6.7; P = 0.008). Cryptosporidiosis was not associated with keeping animals, nutritional status, or parental education, but was apparently more common in breast fed children (OR = 2.7, 95% CI = 1.1, 6.9; P = 0.01), although the proportion of exclusively breast fed children was not measured. Since most of these infections were of short duration, we conclude that transmission of C. parvum can vary dynamically within one city and over short periods of time, and that water-borne contamination may be a substantial influence.