Peripheral blood mononuclear cells (PBMC) from 8 onchocerciasis patients, treated or not with ivermectin, were analyzed for phenotypic cell surface markers. A significant increase (P < .05) in γ/δ T cells expressing the Vδ1 chain compared with normal and endemic controls was detected in all patients. PBMC populations from onchocerciasis patients were not expanded after restimulation with Onchocerca volvulus antigens in vitro, but both Vδ1 and Vδ2T cells from normal donors were increased significantly in response to O. volvulus and Mycobacterium tuberculosis (P < .05), respectively. Frozen sections of all 5 onchocerca nodules tested demonstrated an increased number of CD3⁺ cells in the vicinity of the adult worm, in all cases expressing the α/β T cell receptor and in 2 patients also expressing the γ/δ T cell receptor; 60% of T cells expressed the activation marker Ki67. These data suggest that O. volvulus provides ligands to Vδ1 T cells.