The effect of lambda-cyhalothrin impregnated bed nets and maloprim/placebo was studied in approximately 1,500 children living in 17 villages in a rural area of Sierra Leone, approximately 150 miles south east of Freetown, 30 miles north of the town of Bo. Villages were selected randomly amongst villages with impregnated bed nets and villages with no nets at all. Within these villages, children with ages ranging between 3 months to 6 years were chosen to receive maloprim or a double-blind distributed placebo fortnightly. In the villages randomised to receive nets, all beds have received nets. Malaria morbidity is estimated from weekly active case detection, and the impact on the Anopheles vector is being estimated by indoor spray catching, exit trap catching, human night landing catches and light trap catches. During the first 8 weeks of the intervention there was a significant reduction in slide positive rates, reported fever rates and children with temperature > or = 37.5 degrees C in the villages with impregnated bed nets.