One hundred patients were included in a randomised trial to identify surgical factors inducing malaria and assess the impact of chloroquine prophylaxis in Niamey where resistance to the drug is moderate. Thick smears were positive in 19 patients without prophylaxis (37%) versus 7 under chloroquine (15%). Factors influencing malaria were surgical intervention lasting more than 1 hour and general anaesthesia. Chloroquine prophylaxis was significantly efficient under such circumstances.