Background & Aims: The aim of this study was to evaluate the efficacy of nitazoxanide for the treatment of diarrhea and enteritis associated with Blastocystis hominis as the sole identified pathogen in children and adults from the Nile delta of Egypt. Methods: Two prospective, randomized, double-blind, placebo-controlled studies were conducted. Nitazoxanide 500 mg (as a 500-mg tablet) was administered twice daily for 3 days in patients aged 12 years or older, 200 mg (as 10 mL of an oral suspension) was administered twice daily for 3 days in patients aged 4–11 years, and 100 mg (as 5 mL of an oral suspension) was administered twice daily for 3 days in patients aged 1–3 years. Results: Four days after the completion of therapy, 36 (86%) of the 42 patients who received nitazoxanide showed resolution of symptoms compared with 16 (38%) of 42 patients who received placebo (P < .0001). Thirty-six (86%) of the 42 patients who received nitazoxanide were free of B hominis organisms in each of 3 posttreatment stool samples compared with only 5 (12%) of 42 patients who received placebo (P < .0001). Response rates in patients receiving the tablets and the suspension were identical. Conclusions: These findings suggest that B hominis is pathogenic in some patients and can be treated effectively with nitazoxanide. Alternatively, the possibility that nitazoxanide is effective in treating other unidentified causes of persistent diarrhea and enteritis warrants further study. Supported by a grant from Romark Laboratories, LC, Tampa, FL. Dr Rossignol is an employee of and owns an equity interest in Romark Laboratories, LC, the pharmaceutical company that owns the patent rights related to nitazoxanide