Between 1991 and 1994, 4 annual treatments with ivermectin were offered to the population of an area hyperendemic for onchocerciasis in the province of Bururi (Burundi). In one place, Kinama, the annual coverage ranged between 40 and 49% of the population. Of 151 persons who were examined there in 1987 and again in 1994, 4 months after the last treatment round, 43 took ivermectin all 4 times, 34 took it 3 times, 35 twice and 17 only once. Although the compliance in this population was incomplete and far from regular, the prevalence of infection in a sample of untreated 4 and 5 year-old children dropped from 60.0% in 1987 to 23.7% in 1994 (a reduction of 60.5%, P < 0.01) and their microfilarial load (geometric mean number of microfilariae in scarification slides) from 3.0 to 0.4 (a reduction of 86.7%, P < 0.01). It appears that even a suboptimal mass distribution of ivermectin can reduce the microfilarial load to a level where the transmission of O. volvulus becomes seriously affected.