Interleukin-2 receptor, the complex of IL-2R-alpha and/or IL-2R-beta, is expressed mainly on T-lymphocytes, and the soluble form of IL-2R-alpha (sIL-2R-alpha) has been reported to be detected in the serum of patients with lymphoproliferative disorders or disease characterized by the cellular immune reaction. We measured serum sIL-2R-alpha levels among patients with pulmonary diseases and found that sIL-2R-alpha levels were significantly elevated in patients with active pulmonary tuberculosis (1,327 +/- 209 U/ml) and sarcoidosis (1,037 +/- 115 U/ml) when compared with healthy volunteers (468 +/- 49 U/ml, p less than 0.01). Among patients with pulmonary tuberculosis, the sIL-2R-alpha levels were high in sera from patients with extensive parenchymal lesions on the roentgenogram (2,745 +/- 705 U/ml) and patients with tuberculous pleurisy (2,111 +/- 679 U/ml). In contrast, the sIL-2R-alpha levels in tuberculous patients with minimal lesion (455 +/- 92 U/ml) or moderate lesion (1,082 +/- 189 U/ml) were not significantly elevated when compared with healthy volunteers. After the treatment with antituberculosis agents, serum sIL-2R-alpha levels decreased in accordance with improvement of roentgenographic findings and laboratory data. These results suggest that serum sIL-2R-alpha level may be useful as a monitor for the disease activity in patients with pulmonary tuberculosis.