To study the value of knowing the proportions of bronchoalveolar T cell subsets when predicting the course of pulmonary sarcoidosis, we subjected 31 patients to clinical, physiologic, and radiographic evaluations, with controls, for at least 12 months. Initially, when all patients were untreated, BAL's were performed, and BAL lymphocyte subsets were marked by the following monoclonal antibodies: OKT3 (expressed by all T cells), OKT4 (to mark helper-inducer T cells), OKT8 (to mark suppressor-cytotoxic T cells), and OKIa (to mark Ia antigen-positive, activated T cells). A normal T4/T8 ratio was highly predictive of a favorable course: the conditions of 13 out of 15 patients with normal ratios remained stable or improved, and only 2 of these 15 patients had to be treated because of persistent symptoms. On the other hand, the conditions of 10 out of 16 patients with elevated T4/T8 ratios deteriorated during the follow-up period. The specificity of T cell subsets for predicting deterioration was improved by considering both the T4/T8 ratio and the number of Ia antigen-positive, activated T cells present. Deterioration occurred in 9 out of 11 patients with elevated T4/T8 ratios and elevated levels of activated T cells. These results suggest that the subtyping of BAL lymphocytes may be useful in determining prognosis in pulmonary sarcoidosis.