Twenty-two patients with active pulmonary sarcoidosis had the subpopulations of the lymphocytes in the bronchoalveolar lavage (BAL) fluid measured. All 12 with hypergammaglobulinaemia had a T4/T8 ratio of greater than 2.0 and five of 10 without hypergammaglobulinaemia had a ratio less than 2.0 (chi 2 = 5.18, P less than 0.025). When an increased number of T4 lymphocytes was present in the BAL fluid, there was a significant decrease in the absolute number of T4 lymphocytes in the peripheral blood (chi 2 = 6.84, P less than 0.01). The number of T4 lymphocytes in the BAL fluid correlated with the subsequent response to daily steroid therapy for 2 months, as assessed by improvement in vital capacity (r = 0.89, P less than 0.01).