Abstract: We compared the efficacy of a single dose of ivermectin with that of a standard course of diethylcarbamazine (DEC) for the control of microfilaremia in 60 patients with banrroftian filariasis who had developed recurrent microfiiaremia after each of three or more prior treatments with DEC.The study was done as a randomized,double-blind trial.Complete,but transient clearance of microfilaremia was observed in both treatment groups.One year later,recurrent microfilaremia was present in 7 patients treated with iVermectin and in 5 treated with DEC.Pretreatment levels of microfilaremia were significantly higher in patients who relapsed within one year after treatment than in those who remained amicrofilaremia.Side effects of either treatment were common but mild.Febrile reactions were more frequent in the ivermectin group: while localized reactions consistent with a flare up of acute filarial disease occurred mostly in the DEC group.We conclude that ivermectin is an effective and practical alternative to DEC for treatment of recurrent microfilaremia due to bancroftian filariasis.