The relationship between the level of gamma delta T cells and cellular immunity of T lymphocytes was assessed in the peripheral blood of active sarcoidosis patients and healthy controls. We divided the active sarcoidosis patients into two groups: a group of patients with a normal distribution of circulating gamma delta T cells (group A; n = 11, less than 8.2% lymphocytes) and another group with increased gamma delta T cell levels (group B; n = 11, greater than 8.2% lymphocytes). The proportion and absolute count of CD4+ lymphocytes in group B (28.6 +/- 11.2%, 374.1 +/- 193.8/microliters) were remarkably smaller than control subjects (45.7 +/- 6.8%, 818.3 +/- 290.2/microliters, P < 0.001, P < 0.002, respectively). Group A, however, showed a moderate reduction in CD4+ lymphocytes when compared with controls. Serial measurements of T cell subtypes were performed on five patients in group B. gamma delta T cells and CD4+ lymphocytes were inversely correlated over the observation period which ranged from 2 to 18 months. When peripheral blood T cells were stimulated with PHA or PPD in vitro, the responses were weaker in group B compared with both group A and control subjects. These results suggest that the increase in circulating gamma delta T cells in sarcoidosis closely relates to a defect in cellular immunity which progresses during the disease process.