A prospective randomized, double-blind, placebo-controlled study was conducted in 50 adults and 50 children from the Nile delta of Egypt, to evaluate the efficacy of nitazoxanide in treating diarrhea caused by Cryptosporidium parvum. Nitazoxanide was administered in 500-mg doses twice daily for 3 days in adults and adolescents, in 200-mg doses twice daily for 3 days in children aged 4-11 years, and in 100-mg doses twice daily for 3 days in children aged 1-3 years. At 7 days after initiation of therapy, diarrhea had resolved in 39 (80%) of the 49 patients in the nitazoxanide treatment group, compared with 20 (41%) of 49 in the placebo group (P<.0001). Diarrhea was resolved in most patients receiving nitazoxanide within 3 or 4 days of treatment initiation. Nitazoxanide treatment reduced the duration of both diarrhea (P<.0001) and oocyst shedding (P<.0001).