Syndecan-1 (CD138), an important transmembrane heparan sulfate proteoglycan is expressed in distinct stages of cell differentiation. Although its expression in acute lymphoblastic leukemia (ALL) cells is well known; its function or presence in acute myeloblastic leukemia (AML) cells is still largely unknown. The expression of syndecan-1 was studied in bone marrow biopsies of three patients with AML using electron microscopic immunocytochemistry. Positive expression of syndecan-1 was found in AML cells. These results suggest that syndecan-1 expression is not only a characteristic phenotypic marker for ALL, but is also expressed in AML cells.