The distinction between WM and SMZL may be difficult since both entities share overlapping clinical, immunophenotypic, and histopathologic characteristics. In this context we evaluated whether CD138 expression could be added in the armamentarium of tools used to differentiate these entities. Sixty-nine patients were studied, 47 WM and 22 SMZL. Paraffin-embedded sections of bone marrow biopsies were quantitatively and qualitatively evaluated for CD138 expression. Sixty percent of WM cases expressed CD138 in contrast to 18% of SMZL patients. Intermediate/high intensity of CD138 expression was observed in 47% of WM while it was low in all SMZL patients. Differences between WM and SMZL regarding the intensity and the percentage of CD138 positive cells were both significant (P=.0008 and, .00021 respectively). Moreover, CD138 expression was related to serum IgM levels in WM patients (P=.0006). In conclusion, CD138 expression may constitute an additional aid in the distinction between WM and SMZL.