Assuming an attack rate of 0.1 episodes of clinical malaria (as defined for the primary end-point) per month in children in the control group (a figure based on previous surveys from The Gambia) and a drop out rate during follow up of 15%, 131 episodes was expected in children in the control group followed until the end of one malaria transmission season. Thus, with a total of 1200 children recruited to the trial, the study will have an 80% power to detect at 5% level of significance a 32% reduction in incidence of clinical attacks of malaria in children who receive chemoprophylaxis.