Following a presumptive treatment with 35 mg chloroquine base/kg, 484 Namibian children between 5 months and 5 years of age received 50 mg of proguanil daily for 4 months. They were compared with 268 children living in a very adjacent area who received vitamin tablets after the initial chloroquine medication. Fewer fever episodes were recorded among the children who received proguanil and they were also requiring less presumptive treatments with chloroquine during the period of study, but there were only minor differences in parasite rate between the two groups at the end of the study period. Despite the reduction of morbidity, the required efforts were too large to justify another period of drug prophylaxis.