An 82-year-old Japanese male patient was initially diagnosed with lymphocytosis. His complete blood count revealed a white blood cell count of 30.9×109/l with 81% abnormal lymphocytes. The abnormal lymphocytes included monoclonal clones of CD38+ and CD138+cytoplasmic κ+ and IgG-κ M-protein, which led to the final diagnosis of plasma cell leukemia (PCL). Bortezomib and dexamethasone therapy was initiated, but the patient succumbed to the disease on the 8th day of hospitalization. A cytogenetic examination revealed a t (9;14)(p13;q32) translocation and the Western blotting confirmed high PAX5 expression. Similar to our present case, PCL cases with "lymphocytosis" have been widely reported, which some speculating the involvement of PAX5 overexpression in the pathogenesis. Such cases, including ours, may be classified as a unique group of disorders (PCL presenting as "lymphocytosis"), which requires accurate differential diagnosis and subsequent urgent multidisciplinary intensive treatment.
Keywords: Lymphocytosis; PAX5 gene; Plasma cell leukemia; t(9;14)(p13;q32).