Sarcoidosis is a granulomatous disorder of unknown cause characterized by activation of T-lymphocytes. We here report the use of an enzyme-linked immunosorbent assay for the soluble interleukin-2 receptor (IL-2R) as a measure of T-cell activation in serum samples and bronchoalveolar lavage fluids in 15 patients with active sarcoidosis. The geometric mean (x divided by SEM) value for soluble IL-2R in serum samples from patients with sarcoidosis was 1,110 (x divided by 1.17) versus 224 (x divided by 1.08) U/ml for normal control subjects (p less than 0.001). Detectable levels of soluble IL-2R were present in bronchoalveolar lavage fluids from 10 of 15 patients with sarcoidosis versus only 2 of 36 normal control subjects (p less than 0.001). Levels of soluble IL-2R in serum samples from untreated patients with sarcoidosis correlated with 67gallium lung scanning scores (p less than 0.05) but not with serum angiotensin-converting enzyme concentrations or constituents of bronchoalveolar lavage. In 5 patients, the level of soluble IL-2R in serum samples fell from 1,499 (x divided by 1.20) to 476 (x divided by 1.58) U/ml (p less than 0.05) after 6 wk of successful treatment with corticosteroids, whereas the changes in soluble IL-2R in bronchoalveolar lavage fluids were more variable. These observations suggest that measurements of soluble IL-2R, particularly in serum samples, may reflect disease activity and be clinically useful in the management of patients with sarcoidosis.