The efficacy of the standard formulation of halofantrine hydrochloride has been compromised by the formulation's irregular bio-availability. A micronized preparation of the drug has now been evaluated in the treatment of malaria in northern Tanzania. Overall, 100 patients with mild to moderate Plasmodium falciparum malaria were recruited and treated with the preparation over 18 h. Those weighing > 40 kg were each given three, 500-mg doses and those weighing less were given roughly equivalent doses/kg. The 95 evaluable patients were all successfully treated, with a mean fever-clearance time of 22.5 h (range 4-76 h) and a mean parasite-clearance time of 35.6 h (range 15-66 h). There were no relapses. Abdominal pain was the commonest adverse event reported (22 cases). A single patient died suddenly in the recovery phase; the cause of this event was not determined. Further studies are required to evaluate the pharmacokinetics of the halofantrine formulation under field conditions.