Four groups of eight patients receive escalating doses of oral letrazuril (or placebo). In each group, six patients are randomized to receive letrazuril and two patients receive matching placebo. In the pharmacokinetics determination phase of the study, patients receive a single dose of letrazuril or placebo following a meal. Following a 72-hour blood collection, patients enter the blinded, treatment phase of the study and receive letrazuril or placebo as a single dose daily, after a meal, for 3 weeks. Patients with persistent Cryptosporidium oocysts in their stools at the end of the blinded treatment phase may continue with open-label treatment of letrazuril at the same dose for 4 weeks; the dose may subsequently be escalated every 4 weeks, to a maximum, if oocysts persist. Patients who have Cryptosporidium oocysts eradicated from their stools will discontinue treatment and be followed for 3 months. All patients undergo clinical follow-up at 3 and 6 months.