One hundred and eighteen patients with acute falciparum malaria were randomized into treatment with either intramuscular or oral sulfadoxine-pyrimethamine (Fansidar, Roche) and the results were compared with those from 68 patients treated in parallel with chloroquine. Parasitological cure rate was 97% with oral sulfadoxine-pyrimethamine, 95% with the injection, and only 63% with chloroquine. The time for the disappearance of parasitaemia in sensitive cases was the same with oral and intramuscular sulfadoxine-pyrimethamine and shorter than with chloroquine. Side effects occurred in only 3 of the patients treated with intramuscular sulfadoxine-pyrimethamine compared with 8 of those treated with the tablets and 13 of those treated with chloroquine. The results showed that intramuscular sulfadoxine-pyrimethamine is as effective as, and probably better tolerated than, the oral drug. Increasing failure of response to chloroquine in Nigeria was also demonstrated.