Forty horses having microfilariae of Onchocerca cervicalis in association with dermatitis, alopecia, and pruritus on the ventral midline were given a single IM injection of 0.2 mg of ivermectin/kg of body weight (June to August 1981). Microfilarial counts in the 40 horses ranged from 18 to 42,446 microfilariae/skin snip on the day of treatment, and histopathologic examination of these skin sections indicated a chronic eosinophilic dermatitis. Numerous microfilariae were in the dermis, but there was no consistent relationship between the presence of microfilariae and the severity of the inflammatory reaction. In all 40 horses, skin snips taken 4 to 33 days after treatment were negative for microfilariae, and a marked clinical improvement occurred 2 to 3 weeks after treatment, when the lesion was replaced by healthy skin and new hair. Twenty-four hours after treatment, an edematous reaction occurred on the lower portion of the abdomen of 4 (10%) horses and within the area of the lesion in 6 (15% horses). The reactions disappeared within 24 to 72 hours, irrespective of whether horses were treated with corticosteroids. When further skin samples were taken from 15 horses 4 to 9 months later, 9 of them were free of microfilariae and the 6 others had only low counts (17 to 97). An additional 7 infected horses, treated when there was little chance of reinfection (November to December 1981), were all free of microfilariae at 6 to 10 days and 4 to 5 months after treatment. The marked clinical improvement in all horses after disappearance of microfilariae from the dermis indicates that microfilariae are involved in the cause and pathogenesis of the dermatitis.