Patients are enrolled in groups of 7. Group A receives the lowest dose of NTZ. Groups B - D receive sequentially higher doses. Patients in Groups B - D do not begin therapy until all patients have been enrolled at the preceding dose. For determination of single-dose pharmacokinetics, patients receive a single dose of NTZ on Day 1 and blood samples are obtained over the next 24 hours. Immediately following the 24-hour blood sample collection, patients begin the treatment phase during which they take NTZ for an additional 13 days. Blood samples are obtained again on Day 14 to determine full plasma concentration-time profiles over the dosing interval. At the end of treatment, responders discontinue treatment and nonresponders continue NTZ treatment for an additional 14 days on the same dose. Patients who still fail to respond are provided with an additional month of NTZ. Patients have scheduled visits for clinical and parasitologic efficacy assessments and safety laboratory tests.