T lymphocyte numbers and functions were measured in forty-six patients with alopecia areata and thirty healthy controls. In patients with alopecia areata, lymphocyte reactivity to extracts of scalp and hair follicles was not detected by 3H thymidine incorporation. 3H Thymidine incorporation by lymphocytes cultured with PPD, Varidase and C. albicans was significantly reduced in cells from patients compared with controls and correlated with the extent of hair loss and the presence of antithyroid antibodies. Decreased lymphocyte responses to PPD and Varidase were not due to the presence of suppressor monocytes because removal of monocytes with carbonyl iron did not increase lymphocyte reactivity. However, responsiveness to C. albicans was augmented by removal of monocytes. T cell numbers were significantly lower than in controls (P less than 0.005); patients with auto-antibodies had fewer T cells than patients without them. The relationship of reduced T cell function and auto-immunity to alopecia areata is discussed.