In an area in which brugian filariasis is endemic, when cytokine levels were analyzed as a function of clinical status comparing those who were asymptomatic and amicrofilaremic with those who were microfilaremic, it was found that both interferon (IFN)-γ and interleukin (IL)-5 were suppressed in microfilariae carriers (P < .01 and P < .001, respectively), but IL-4 was unabated. Age had a significant effect on cytokine production in both groups. In asymptomatic amicrofilaremic subjects, IL-4 production was high in young persons and decreased with age, whereas in microfilaremic subjects, IL-4 increased significantly with age. Conversely, IFN-γ showed a tendency to increase with age in asymptomatic amicrofilaremic subjects but not in microfilaremic subjects. IL-5 decreased significantly with increasing age in both asymptomatic amicrofilaremic and microfilaremic groups. These results indicate that the length of exposure to and infection with filarial parasites can each exert a substantial effect on the cytokine response profiles of host T cell populations.