We measured serum levels of soluble Fas(sFas) in 68 patients with hematological malignancy by using sandwich ELISA and detected bone marrow mononuclear cell membrane Fas(mFas) positive cells in 46 patients with acute leukemias by immunohistochemical technic. It was found that sFas levels were significantly higher in acute lymphoblastic leukemia(ALL) patients(median 10.48 +/- 5.89 ng.ml-1) and non-Hodgkin's lymphoma(NHL) patients(median 18.26 +/- 16.47 ng.ml-1) than those in healthy donors (median 0.92 +/- 0.88 ng.ml-1) (P < 0.05). Elevated sFas levels were correlated with the disease stage and progress, but not with other clinical parameters. According to the follow-up of five ALL patients, their serum sFas levels decreased after complete remissions. We conclude that sFas has prognostic value and is an effective clinical parameter with ALL or NHL patients. In addition, sFas levels in the sera of acute non-lymphoblastic leukemia patients were also found to be unchanged compared with those in the normal control.