To study the relationship between the level of the soluble L-selectin (sL-selectin) in plasma and surface L-selectin expression on leukemic cells and episode and state of illness in acute leukemia patients, the plasma level of sL-selectin was measured by a sandwich enzyme-linked immunosorbent assay, and the expressions of surface L-selectin and its gene (lyam-1) were detected by immunohistochemistry and RT-PCR. The results showed that the levels of sL-selectin were significantly higher in untreated and therapy-resistant acute leukemia patients, and expression of L-selectin mRNA and cell surface L-selectinin in untreated and NR patients were significantly lower than that in CR patients and control group (P < 0.05). The plasma levels of sL-selectin were significantly increased in patients with splenomegaly and hepatomegaly (extramedullary infiltration). The levels of sL-selectin were related to the clinical course of the acute leukemia patients. A significant correalation existed between expressions of L-selectin mRNA and surface L-selectin in acute leukemia patients (gamma = 0.782, P < 0.05). It is concluded that expression of L-selectin gene was down-regulated in level of mRNA and protein in acute leukemia patients and both changes were highly correlated. Monitoring of the plasma level of sL-selectin is possibly useful for early diagnosis of relapse and extramedullary infiltration in acute leukemia.