An increased plasma cell count in the bone marrow occurs in a subgroup of patients with acute myeloid leukemia (AML). The pathogenic mechanism for this plasmacytosis is unclear. In this report we describe two patients with AML and plasmacytosis who shared some features of their diseases. The morphological subtypes were AML M4 and M4eo; the leukemias were secondary to cytotoxic pretreatment, and complex cytogenetic changes were found in the leukemic cells of both patients. There was a marked increase in the number of bone marrow plasma cells in both cases and no monoclonal immunoglobulin was detectable. The IgH-CDR3 gene scan depicted a monoclonal IgH rearrangement in the bone marrow cells of one patient. Analysis of the cytokine production of the leukemic cells showed a high production of IL-6 of the leukemic blast cells in the in vitro cell culture and a high cytoplasmic IL-6 in the leukemic cells as revealed by immunocytology. We describe the clinical picture of a type of secondary AML with FAB M4 morphology associated with bone marrow plasmacytosis. We suggest that paracrine growth stimulation of plasma cells by paraneoplastic IL-6 production of the leukemic blast cells contributes to the plasmacytosis observed in patients with AML.