On the basis of previous experience in the study area, it was estimated that the incidence of malaria in children in the control group would be about 0.3 attacks during the malaria transmission season. On this basis a study with 600 children in each arm would have 80% power at the 5% level of significance to detect a reduction in the incidence of malaria of 25% in the children who received BCG. A trial of this size would have 80% power to detect a reduction in the prevalence of parasitaemia of 22% at the final cross-sectional survey assuming a prevalence in the control arm of 35% asexual P. falciparum parasitemia, an assumption based on the results of a previous survey.