To determine the effectiveness of single oral dosages of ivermectin ranging between 20 and 200 micrograms/kg and to make detailed observations of both the kinetics of parasite killing and the adverse reactions induced by treatment, the present double-blind study on ivermectin treatment of lymphatic filariasis caused by Wuchereria bancrofti was undertaken with 43 microfilaremic patients in Recife, Brazil. Follow-up at one year indicated equivalent efficacy for the 20-, 100-, and 200-micrograms/kg drug dosages in reducing microfilaremia to geometric means of 13-25% of pretreatment levels. Adverse clinical reactions (predominantly fever, headache, weakness, and myalgia) occurred to some degree in almost all patients but generally lasted only 24-48 hr and were easily managed symptomatically. Adverse reactions were significantly milder in those receiving the lowest (20 micrograms/kg) ivermectin dose, and they were significantly correlated with individuals' pretreatment microfilaremia levels in all groups. Posttreatment eosinophilia was a regular feature of the response to treatment, with the magnitude and kinetics also proportional to pretreatment microfilarial levels. Transient pulmonary function abnormalities (16 of 42, 38%), liver enzyme elevations (10 of 43, 23%), and hematuria (9 of 42, 22%) developed posttreatment, but all cleared without significant complications. The results indicate that W. bancrofti from Brazil is similar to strains of the parasites studied elsewhere in susceptibility to ivermectin, that the drug's systemic adverse reactions are essentially those resulting from parasite clearance, and that the intensity of these reactions can be significantly reduced by using the low (20 micrograms/kg) dose of ivermectin. This detailed dose-finding study provides information necessary for developing optimal regimens to treat bancroftian filariasis with ivermectin either alone or in combination with other medications.