There is a small population of peripheral T cells bearing the gamma delta T-cell receptor, which may be involved in the defense against invading microorganisms and tumor cells. The present study was designed to evaluate the levels of gamma delta T cells in patients with pulmonary tuberculosis, bacterial pneumonia, chronic lower respiratory tract infection, lung cancer, and normal control subjects with or without old tuberculous lesion. The results showed that only patients with tuberculosis had significantly increased proportions of peripheral blood gamma delta T cells. This study suggests that the increased proportions of gamma delta T cells in tuberculosis could be related to T-cell activation by Mycobacterium tuberculosis, although it remains to be investigated which components of mycobacteria are the major ligands for gamma delta T cells.