Plasma cell leukemia (PCL) is a neoplastic disorder of plasma cells of which there are 2 forms, primary PCL and secondary PCL, the latter occurring in patients with a history of plasma cell myeloma. We describe 4 patients with primary PCL. In all cases, the bone marrow aspirate smears and biopsy specimens demonstrated a diffuse infiltrate of atypical plasma cells that were difficult to classify using morphologic criteria alone. Immunophenotypic studies showed that each case was positive for plasma cell-associated antigens (cytoplasmic immunoglobin, CD38, or CD138) and negative for CD20. Of 4 cases, 3 had complex karyotypes, including the t(11;14)(q13;q32) or del(11)(q13). Despite chemotherapy and the use of novel therapeutic agents and stem cell transplantation, all 4 patients had short survival.