Serum levels of soluble interleukin-2 receptor (sIL-2R) have been evaluated in 33 patients with chronic lymphoproliferative disorders of T-lymphocytes, including 12 T-helper phenotype chronic lymphocytic leukemias (Th-CLL) and 21 lymphoproliferative diseases of granular lymphocytes (LDGL). All Th-CLL cases were negative for antibodies against type I human T-leukemia virus (HTLV-I). Serum levels of sIL-2R were significantly increased in patients with Th-CLL with respect to controls (P less than 0.02) and this increase was related to the clinical course of the disease. In fact, patients with rapidly progressive disease (mean survival, 11.6 +/- 3 months) showed significantly higher concentrations of sIL-2R than patients with less aggressive disease (mean survival, 39.5 +/- 5 months) (16,223 U/ml +/- 5612 versus 1447 U/ml +/- 817; P less than 0.05). A significant positive correlation was found between sIL-2R concentrations and the number of CD4-positive cells (r = 0.64; P less than 0.05). These data point to the possible use of sIL-2R levels as a marker of active malignancy in patients with Th-CLL. Patients with LDGL showed increased sIL-2R values (721 U/ml +/- 112) with respect to controls (334 U/ml +/- 28; P less than 0.005). However, the sIL-2R levels were lower than those detected in Th-CLL patients (P less than 0.05). Among the different correlations the authors tried to establish, the only observed difference was found between serum sIL-2R levels in the group of CD3+ LDGL patients with respect to CD3- LDGL cases (P less than 0.05).