One hundred thirty nonimmune subjects living in Yaoundé, Cameroon, completed an 18-month prospective study on the efficacy and safety of weekly chloroquine and daily proguanil (chloroguanide) (Ch-P) in malaria prevention. A total of 9 of 78 Ch-P-treated subjects and 26 of 52 subjects who received no prophylaxis contracted Plasmodium falciparum infection during this period (P less than 0.00005). These two groups were comparable for demographic parameters and degree of exposure. Clinical manifestations were of similar severities in the two groups, but parasite counts were significantly higher in the subjects who received no prophylaxis (P less than 0.00005). Side effects of prophylaxis were frequent (31%), minor, and related to chloroquine, and they usually resolved within 4 to 6 weeks. Prolonged administration of Ch-P is safe and effectively prevents P. falciparum malaria in an endemic area with a high prevalence of chloroquine resistance.