We report an unusual diagnostic problem in a 35 year old woman with bilateral breast lumps. The patient first presented in August 1998 with a two week history of fever, tooth infection, and easy bruisability. Her full blood count showed severe pancytopenia (total white blood cell count, 1.0 × 10⁹/litre; neutrophil count, 0.1 × 10⁹/litre; haemoglobin, 66 g/litre; platelet count, 22 × 10⁹/litre). Her past medical history was unremarkable. She had no causative medications, chemical exposure, or viral symptoms. Flow cytometry for paroxysmal nocturnal haemoglobinuria was negative. A bone marrow biopsy showed a hypolastic marrow consistent with aplastic anaemia. She had no response to standard doses of antithymocyte globulin, methylprednisone, and cyclosporine and underwent a sex mismatched sibling allogeneic transplant. She had essentially no graft versus host disease, and …