Summary We conducted two randomized clinical trials to determine the in vivo efficacy of amodiaquine and sulfadoxine/pyrimethamine in treating Plasmodium falciparum malaria. Seventy-five patients under the age of 10 years in Kibwezi, Kenya, and 171 patients in Kigoma, Tanzania, were enrolled for treatment. Due to loss of eight patients in Kibwezi and 37 in Kigoma to follow-up, we used best and worst case scenarios for the parasitological response. The in vivo sensitivity of Plasmodium falciparum to amodiaquine was 75% (no loss to follow-up) in Kibwezi and ranged from 85% in the best to 65% in the worst case scenario in Kigoma. The sensitivity to sulfadoxine/pyrimethamine was 70% to 88% in Kibwezi and 65% to 89% in Kigoma. R1 resistance to amodiaquine was 22% in Kibwezi and varied from 6% in the best to 26% for the worst case scenario in Kigoma. The R1 resistance to sulfadoxine/pyrimethamine was 5% to 23% in Kibwezi and 2% to 26% in Kigoma. R2 resistance was 3% for amodiaquine and 7% for sulfadoxine/pyrimethamine in Kibwezi and 9% in Kigoma for each treatment group. There was no statistically significant difference between treatment groups at either study site, except for a slight difference in R1 resistance in the best case scenario, Kibwezi, in favour of S/P. Although both amodiaquine and sulfadoxine/pyrimethamine resistance seems to be increasing, these antimalarials are still effective in parasite clearance.