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Since 1985, chloroquine resistance (CR) became evident in Cameroon, and rapidly spread out over the country. Despite such a situation, chloroquine is still the first antimalarial drug. CR follow up was mostly carried out by in vivo tests among schoolchildren. In this population, of a mean 9 yrs of age, a CR rate of 30 to 45% was reported. However, this may not show the true level of CR, because of the interaction of immunity in controlling parasite growth. Therefore, in southern Cameroon, we studied the activity of 25 mg/kg over 3 days of chloroquine and amodiaquine in treating asymptomatic children ranging from 1 to 15 yrs, with parasite density > 1000/microl. Following chloroquine treatment, children with treatment failure by D7 in the 0-2, 3-4, 5-9 and 10-15 yrs age groups were : 26-37 (71%), 9/21 (43%), 24/43 (56%) and 12/30 (40%). Four infections were RIII, all in the 0-2 yrs group. 41 of the 60 children with a D7 negative blood smear were seen again at D14 : 19 of these (46%) were positive. Following amodiaquine, children with treatment failure by D7 were : 3/28 (11%), 2/20 (10%), 1/17 (6%) and 0/6. No RIII level was observed. 44 of the 65 children with D7 negative blood smear were seen on D14 : 7 (16%) had a positive blood smear, 5 being in the 0-2 yrs group. The prevalence of CR in southern Cameroon is high, especially in young children. The prevalence and level of CR decreased with age. Amodiaquine is effective > 89% in all age groups, and might be considered as a first line treatment in young children. Medical practitioners must be provided with information on resistance within groups having not yet acquired premunition. (Résumé d'auteur)