The recognized high mortality from measles in Africa is considered to be partly due to the flare-up of concomitant malaria infection. In 1987 there was a measles epidemic in the Rufiji Delta, Tanzania, in spite of recent vaccination campaigns. A comparative study was therefore conducted on the densities of malaria parasites in children during the acute stage of measles (67 consecutive cases, aged 5 months-19 years). The period of study was March-June, the peak season for malaria transmission. For each measles patient, a blood film was concomitantly taken from an asymptomatic age-matched child from the same village. Of 67 children with measles, 17 (25%) had parasitaemia ranging from 8 to 2480 parasites microliter-1 blood. Out of 67 asymptomatic control children 59 (88%) had parasitaemia ranging from 8 to 3400 parasites microliter-1 blood. This study indicates that malaria densities were lower during the acute stage of measles than in healthy children. The contribution of malaria to mortality in children with acute measles may be questioned.