It was assumed on the basis of previous information that children in the control group would experience an average of 0.4 clinical attacks of malaria during the malaria transmission season. On the basis of this assumption, a trial with 540 children in each arm would have 80% power at the 5% level of significance to detect a 30% reduction in the incidence of clinical attacks of malaria and a 90% power to detect a 40% reduction including an allowance for a 10% drop-out during the surveillance period.