Serum soluble interleukin-2 receptor (sIL-2R) levels were determined in patients with chronic myeloproliferative disorders (CMPD): 18 with chronic myelogenous leukemia in chronic phase (CML in CP), seven with CML in accelerated phase (AP) or blastic crisis (BC), six with polycythemia vera (PV), eight with essential thrombocythemia (ET), one with primary myelofibrosis (PMF), and 50 controls. The mean (+/-S.E.M.) levels were higher in CMPD than in controls (CML in AP or BC, 2693 +/- 694 U/ml, P < 0.0001; CML in CP, 792 +/- 63 U/ml, P < 0.0001; PV 553 +/- 89 U/ml, P < 0.05; ET, 449 +/- 56 U/ml; PMF, 628 U/ml vs. controls, 395 +/- 25 U/ml). Patients with CML in CP had significantly higher serum sIL-2R levels than patients with ET (P < 0.005), and levels were markedly elevated in AP and BC (P < 0.001). Serum sIL-2R levels were positively correlated with WBC count and lactic dehydrogenase in CMPD, and in CML in CP. Serum sIL-2R levels in CMPD were negatively correlated with RBC and platelet counts. Serum sIL-2R levels were significantly lower in patients with CML in CP who showed a cytogenetic response after interferon (IFN) therapy than in those who showed no response (P < 0.05). These findings suggest that a high serum sIL-2R level reflects the leukocyte growth in CMPD and is useful both for differentiating CML from other CMPD and for predicting the response to IFN therapy in CML.