Onchocerciasis ('river blindness') has for several centuries been the scourge of people living in certain areas of the world where the disease is endemic. The treatment available up to 10 years ago, diethylcarbamazine, had very severe secondary effects. The availability of ivermectin--a well tolerated and highly effective microfilaricidal drug--has completely changed this scenario. Ivermectin is now considered to be the drug of choice for the treatment of onchocerciasis. The prognosis for people with onchocerciasis has changed greatly. It is now possible to avoid the heavy infection loads seen previously, and patients, especially expatriates, may have their symptoms relieved by treatment. Ivermectin, used in mass treatment, may also improve the epidemiological situation, reducing the level of microfilariae in the skin of infected people and thus reducing the source for vector infestation. However, the treatment has to be repeated because the drug has no macrofilaricidal effect. Research today is focused on the finding of a drug able to destroy the adult worms that go on producing microfilariae for the length of their lives.