The primary trial end-point was the incidence of malaria in children in the two study groups. Malaria was defined as the presence of (a) fever or a history of fever,(b) no other obvious cause for the fever and (c) the presence of asexual stage Plasmodium falciparum asexual parasitemia of any density. For a second definition, a parasite density of 5,000 parasites per ul or >, the estimated fever threshold for the study area, was required.