The treatment of onchocerciasis with diethylcarbamazine and suramin is associated with an exacerbation of ocular disease. This occurs more frequently in patients with severe onchocercal ocular disease. We assessed ocular changes following ivermectin treatment in 39 patients with severe ocular onchocerciasis. Patients were treated with 100, 150, or 200 micrograms/kg of ivermectin at either 1 or 2 year intervals and followed for 3 years. There was no evidence for an acute exacerbation of either anterior or posterior segment eye disease, and there was a marked improvement in ocular status in the group as a whole. There was a significant decrease in onchocercal involvement which was maintained for at least 3 years. Annual treatment with ivermectin can be used safely in patients with severe ocular disease and is beneficial over a prolonged period of time.