To examine whether serum levels of soluble interleukin-2 receptor (sIL-2R) may be a good marker of acute graft-versus-host disease (aGVHD), they were determined weekly in 56 patients receiving bone marrow transplantation (BMT). Because of wide variation in the pre-transplant sIL-2R levels (from 135 to 1918 IU/ml), we used a sIL-2R index in this study by comparing the peak levels with the pre-transplant levels. In agreement with previous reports, there was a significant correlation between the grade of aGVHD and the maximal sIL-2R index. The maximal sIL-2R index was 4.66 in patients with grade I to IV aGVHD, whereas it was 2.68 in patients without GVHD. This marker may be useful for monitoring the status of aGVHD. However, it was interesting that sIL-2R levels were elevated from the time of transplantation until the third week even in patients without GVHD or those who received autologous transplantation. Until the third week, no significant differences were observed in sIL-2R index between these patients and those who developed aGVHD during their clinical courses. After the fourth week, a higher sIL-2R index was observed in patients with aGVHD than in the other patients. Some factors other than GVHD contribute to the elevation of serum sIL-2R levels, and we should recognize the limitations of the measurement of this cytokine.