CD44 variant isoforms (CD44v) have been shown to be important factors in adverse prognosis in hematological malignancies. To investigate whether CD44 expression is associated with malignant transformation in multiple myeloma, RNA and protein expression of CD44 standard (CD44s) and CD44v4, v6, v9, v10 containing isoforms was compared on bone marrow plasma cells from normal individuals and myeloma patients at different stages of disease. CD44s protein expression is strongly decreased on myeloma plasma cells and non-malignant B cells in affected bone marrow of myeloma patients, while no differences in CD44s expression were found between blood B cells from normal individuals and myeloma patients. CD44v isoforms were expressed on plasma cells in the majority of normal and myeloma samples analyzed. CD44v9 and v10 containing isoforms were differentially expressed on bone marrow plasma cells from normal individuals (predominantly CD44v9+v10+) and myeloma patients with stable (predominantly CD44v9-v10+) or progressive (predominantly CD44v9+v10- disease. Normal and myeloma plasma cells contained CD44 mRNA transcripts consisting of multiple CD44v exons. In addition, CD44v9 positive myeloma cells carried large CD44 transcripts. These results imply that detection of CD44v isoforms may be a valuable diagnostic tool for monitoring myeloma disease progression and response to treatment.