A study was carried out in southeastern Gabon to evaluate the tolerance and efficacy of single high doses of ivermectin in 31 Loa loa-infected subjects with low-to-moderate parasitemia (7-7,700 microfilaria/ml). The first group of 16 subjects received 300 micrograms/kg of ivermectin and, seven days later, a second group of 15 received 400 micrograms/kg. Complete clinical and biological monitoring was carried out during the first 10 days post-treatment and again after one and three months. All subjects continued with their usual activities during the study. The clinical tolerance of treatment was very good, and except in one case, only mild adverse reactions were observed, with pruritus being the most common symptom. There were no significant changes in blood or urine function test results or in hematologic results, except for a pronounced eosinophil reaction. The 400 micrograms/kg dose of ivermectin equaled or surpassed in tolerance that of 300 micrograms/kg dose. After treatment, L. loa microfilaremia decreased rapidly to less than 9% of the pretreatment value by day 10. This decrease was enhanced with the 400 micrograms/kg dose, although differences between the two groups diminished slightly with time. At 100 days post-treatment, the microfilaremia was still at less than 10% of the initial values in the two groups, which may indicate an effect of ivermectin on the adult worms.