Two Americans, after returning from Africa, developed swelling of a limb. Both patients had pruritus of the swollen limb, enlargement of the regional lymph nodes, transient discrete macular or papular eruptions, eosinophilia, positive serologic findings from tests for filarial antibodies, and microfilariae of Onchocerca volvulus in the skin. Subcutaneous nodules, generalized papular rash, and ocular changes were absent. Pretreatment biopsy specimens of skin revealed eosinophils around vascular channels, focal edema of the dermal collagen, and in one patient a degenerating microfilaria surrounded by the granules of degenerating eosinophils. These changes were intensified following administration of diethylcarbamazine. We think the swelling and the marked inflammatory response was a hypersensitive reaction to an antigenic product of dying microfilariae. Serum immunoglobulins were measured in one patient, and only IgE was found to be elevated. An attempt to detect immunoglobulins, including IgE, in the skin with a direct immunofluorescence technique was unsuccessful.