BACKGROUND: Plasma cell myeloma (PCM) has been sporadically reported to occur simultaneously or subsequently to mature B lymphoproliferative disorders (LPDs), predominantly chronic lymphocytic leukaemia (CLL).METHODS: We describe the clinical and laboratory findings of a 69-year-old male patient who developed plasma cell leukaemia (PCL) 8 years after an initial diagnosis of a low stage CD5+ B LPD and 3 years after treatment for LPD.RESULTS: The transition from a clinically indolent B LPD to an aggressive PCM was documented by bone marrow (BM) biopsy, while flow cytometric (FC) immunophenotyping conferred additional information by disclosing the co-existence of both disorders in BM and the presence of abnormal monotypic PCs in peripheral blood above PCL levels. Phenotypic findings suggested a discrete clonal origin of the two disorders.CONCLUSIONS: This report of PCL development in a patient with residual CD5+ B LPD, emphasizes the need for comprehensive diagnostic evaluation of such cases and scrutiny of their aetiological relationship, including FC immunophenotyping due to its high analytical sensitivity and multiparametric capacity compared to morphology or immunohistochemistry alone. © 2017 International Clinical Cytometry Society.