A 61-year-old woman with aplastic anemia was admitted to our hospital in October 2009 because of fever and abdominal pain. She had been treated with cyclosporin A without showing any effect. On admission, uterine cancer was diagnosed and the left uterine appendages were swollen. She was treated with cefepime for febrile neutropenia without effect, and left-sided adnexitis was diagnosed. After cefepime was changed to meropenem, marked plasmacytosis was observed in the peripheral blood (23%) and bone marrow (79%) with the appearance of skin eruption. Although the plasma cells were morphologically abnormal, the cytoplasmic immunoglobulin light chain deviation was not detected by flow cytometric analysis, and M protein was not found by serum immunoelectrophoresis. She was diagnosed with reactive plasmacytosis and treated with dexamethasone. The drug eruption and plasmacytosis improved soon after starting the treatment. Although reactive plasmacytosis is observed with a variety of conditions, including infection, neoplasms, autoimmune disorders, and hemolytic anemia, it has not been reported to accompany drug eruption. Reactive plasmacytosis is sometimes not possible to distinguish from plasma cell neoplasms on morphology alone and needs to be diagnosed comprehensively by using flow cytometric analysis and immunohistochemical evaluation.