We examined the plasma soluble interleukin-2 receptor (sIL-2R) level in 80 subjects with myelodysplastic syndromes (MDS) and analyzed its correlation with hematologic/immunologic parameters and the subsequent clinical course. Compared with low-risk MDS (refractory anemia (RA) and RA with ringed sideroblasts) and normal individuals, the plasma sIL-2R level was significantly elevated in high-risk MDS (three other MDS subtypes and acute leukemia following MDS) patients. There was a significant negative correlation between the plasma sIL-2R level and the absolute counts of T and natural killer cells. Furthermore, the plasma sIL-2R level showed a significant positive correlation with the total cell mass and blast mass in particular, in the marrow, but not with the absolute count of IL-2Ralpha-chain-positive lymphocytes in the circulation. Fourteen of our 40 low-risk MDS subjects developed at least one of the following events during the follow-up period: erythrocyte transfusion dependence, infections requiring hospitalization, disease progression or MDS-related death. The plasma sIL-2R level was significantly higher in these patients than in event-free low-risk cases. By logistic regression analysis of various parameters in the 40 low-risk subjects, the plasma sIL-2R level was identified as a valuable independent parameter for predicting the development of events. Based on these findings, we hypothesize that the sIL-2R plays a role in the development of morbidity and mortality in MDS by inducing immunologic dysfunction.