We report a case of dengue fever with plasma cells in the blood (3980 per cubic millimeter) and bone marrow (30%) in a 55-year-old woman hospitalized for fever, arthralgias and thrombocytopenia (66,000 per cubic millimeter) on returning from the West Indies. Serological testing confirmed the diagnosis. Plasmacytosis is rare in dengue fever and its frequency and correlation with the different forms of the disease remain to be determined.