Pancytopenia is a rare complication of the thionamide therapy reported secondary to aplastic anemia, the bone marrow being invariably hypocellular. We present a case of a 16-year-old female with Graves' disease who presented with massive bone marrow plasmocytosis mimicking multiple myeloma. The patient had already been on methimazole for a month when she was admitted to the Pediatric Unit with the diagnosis of sepsis. CBC revealed pancytopenia. Bone marrow aspirations showed hypocellular-normocellular bone marrow, 98% of plasma cells. At that time, MMI was discontinued and the patient was started on broad-spectrum antibiotics, dexamethasone, and G-CSF. Bone marrow aspiration day +4 still showed hypo-normocellular marrow, with remaining 6% plasma cells. Myeloma screen was negative; ANC >1,000 at day +7, platelets >50,000 at day +24. Twenty-four months after patient's discharge, her clinical condition, CBC, and bone marrow remained normal. To our knowledge this is the first report of pancytopenia due to MMI, where the usual hypoplasia found is replaced by massive plasmocytosis.