In recent years, the soluble Interleukin-2 Receptor (sIL-2R) has gained recognition as a valuable marker of in vivo activated immune functions in a variety of diseases. We studied sIL-2R levels in patients with cytomegalovirus (CMV) disease, and/or graft versus host disease (GVHD) following bone marrow transplantation (BMT). Our study included 36 patients after T-cell depleted allogenic BMT and 11 healthy controls. Mean sIL-2R serum levels were significantly higher after BMT than before (1273 u/ml vs. 629 u/ml, respectively, p < 0.007). In the patients who developed CMV disease, with or without GVHD, mean sIL-2R levels increased significantly (2866 u/ml p < 0.004); there was a drop after recovery (1949 u/ml), but not a return to pre-CMV onset levels. Similar elevated sIL-2R levels were found in patients during CMV disease only, GVHD only, or both. In patients who developed GVHD, sIL-2R levels were positively correlated with the severity of GVHD (Pearson's correlation coefficient .8322, p < 0.003). We conclude that sIL-2R may serve as a valuable nonspecific marker for the presence of CMV disease and severity of GVHD following T-lymphocyte depleted BMT.