Soluble interleukin-2 receptor (sIL-2R) is a marker of T lymphocyte activation. We measured the amount of serum sIL-2R in 35 patients with active tuberculosis before the initiation of antituberculous treatment. Twenty had pulmonary parenchymal lesion, 8 had tuberculous pleural effusion, and 7 had tuberculous lymphadenitis. The serum sIL-2R values were markedly elevated in patients with pulmonary tuberculosis (parenchymal lesion and pleural effusion) compared with patients with tuberculous lymphadenitis (2,612 +/- 536 versus 538 +/- 121 U/ml, p = 0.023), old, inactive tuberculosis (335 +/- 23 U/ml, p = 0.001), and normal control subjects (376 +/- 38 U/ml, p = 0.001). No significant difference was found between patients with parenchymal lesion and those with tuberculous pleural effusion. There was a positive correlation between serum sIL-2R values and the extent of disease on chest radiograph (r = 0.58, p less than 0.001). We conclude that the amount of sIL-2R may be a useful marker of disease activity and extent of involvement in patients with active tuberculous lesions.