Active pulmonary sarcoidosis is characterized by the alveolar accumulation of activated helper T lymphocytes that are spontaneously releasing IL 2 and proliferating at an enhanced rate. In this regard, sarcoidosis represents a "model" human disorder to test in vivo the known in vitro action of corticosteroids on suppressing the activated IL 2 gene. Comparable groups of patients with active sarcoidosis were prospectively evaluated with no therapy or treated with corticosteroids. Over 3.2 +/- 0.4 mo, the untreated group had no significant change in spontaneous lung T cell release of IL 2 or spontaneous proliferation. In contrast, over the same period, the treated group had marked reduction of spontaneous lung T cell release of IL 2 and proliferation (p less than 0.01, all comparisons before therapy). Furthermore, Northern analysis of lung T cell RNA before therapy demonstrated IL 2 mRNA transcripts, whereas no IL 2 transcripts were observed during therapy. These observations are consistent with the concept that directly, or indirectly, corticosteroids are capable of suppressing the IL 2 gene in activated T lymphocytes in vivo.