Key Clinical Message
Monocytosis is often profoundly implicated in chronic myelomonocytic leukemia (CMML). We diagnosed a 63-year-old woman with CMML involving heterochronological systemic polyserositis with infiltrating benign monocytes. The patient's pleural effusion was eliminated completely with corticosteroid therapy. We speculate that our case represents an immunological reaction with activated monocytes triggered by infection.