AIMS: To investigate whether plasma cell expression of early B cell, late B cell/preplasma cell, T cell, and myelomonocytic antigens or myeloma associated lymphocytic infiltrates correlated with prognosis in bone marrow biopsy specimens of patients with multiple myeloma. METHODS: Bone marrow biopsy specimens of 23 patients with multiple myeloma were investigated for plasma cell expression and interstitial lymphocyte expression of T cell related antigen CD45RO (UCHL-1). RESULTS: Eight patients showed plasma cell expression of CD45RO and 16 showed increased tumour infiltrating CD45RO positive lymphocytes, which were correlated with poor survival by multivariate analyses (p = 0.005 and p = 0.04, respectively). B cell antigens (MB2, CD20) but no T cell specific antigens (CD3) or T cell receptor gene rearrangements were expressed by plasma cells in CD45RO positive myelomas. Of 16 patients with myeloma who had increased tumour infiltrating CD45RO positive lymphocytes, four had interstitial lymphocyte expression of B cell antigens and two had interstitial lymphocyte expression of the T cell specific antigen CD3. CONCLUSIONS: The recognition of plasma cell expression of CD45RO and increased interstitial CD45RO lymphocytes in bone marrow biopsy specimens of patients with multiple myeloma is an adverse prognostic finding not indicative of an aberrant T cell phenotype or genotype; it is consistent with B cell/pre-plasma cell antigen expression by myeloma cells and their lymphocytic precursors.