Chloroquine, in a single dose of 10 mg of base/kg, was given orally to Togolese children <5 years of age as primary therapy for Plasmodium falciparum malaria. A simplified World Health Organization in vivo method was used, as was a sequential analysis procedure for determining if the drug trial was a success or failure. A total of 178 children in 3 regions were treated; 174 (98%) responded successfully, which required a ≥75% reduction in parasites by day 2 and elimination of parasites by day 7. All 4 failures had low blood levels of chloroquine and desethylchloroquine at day 7. A single dose of chloroquine for treating malaria can be considered for those areas of Africa where the efficacy of such therapy is documented, and where an antimalarial drug sensitivity monitoring system is operating.