Soluble interleukin-2 receptors (sIL-2R) are elevated in various disorders involving the activation of T cells. We measured serial serum concentrations of sIL-2R in 30 patients receiving allogeneic BMT to evaluate the usefulness of sIL-2R as a parameter for acute GVHD. In the 17 patients who developed acute GVHD, the sIL-2R concentration rose significantly on day 3 following transplantation, preceding the occurrence of acute GVHD. This change was not seen in the 13 patients without acute GVHD. The serum concentration of sIL-2R decreased as the acute GVHD subsided. The peak concentration of serum sIL-2R correlated with the severity of the acute GVHD. Simultaneous measurement of tumor necrosis factor alpha (TNF alpha) showed a significant rise in patients with acute GVHD, that became evident earlier than the sIL-2R elevation. TNF alpha concentrations also decreased following treatment of the acute GVHD. However, significant rise in TNF alpha were also seen in the early phase of allogeneic BMT in patients who did not develop acute GVHD. Our data suggest that the serum concentrations of sIL-2R as well as TNF alpha might reflect the severity of acute GVHD, and that the serum sIL-2R concentration might be a sensitive and practical indicator for acute GVHD.