To determine the pharmacokinetic profile of single doses of letrazuril in patients with AIDS-related cryptosporidial diarrhea; to determine the dose proportionality of single escalating doses of letrazuril; to determine steady-state concentrations of letrazuril; to evaluate the safety and efficacy of escalating doses of letrazuril, compared with placebo, for patients with AIDS-related cryptosporidial diarrhea.