Azithromycin (a macrolide-like antibiotic) has antimalarial effects in vitro and in animal models. In the course of a randomised trial of trachoma control we examined the effects of azithromycin on parasite and spleen rates in the population aged 5-14 years from eight villages in the Farafenni study area in The Gambia, West Africa. The entire population of four treatment villages received three doses of azithromycin 20 mg/kg weekly (days 1, 8, and 15) and four control villages received daily tetracycline eye ointment topically (days 1-42). Among 226 children studied before treatment and at day 28, azithromycin reduced the proportions with Plasmodium falciparum parasites (rate ratio 0.56, 95% confidence interval 0.44-0.71; p < 0.0001), with palpable spleens (RR 0.50, 95% CI 0.36-0.70; p < 0.0001), with febrile parasitaemia (RR 0.45, 95% CI 0.27-0.75; p < 0.01), and with P malariae infection (p < 0.001). This effect was related more to resolution of parasitaemia than to prevention of new infections.