Extramedullary plasmacytoma of the liver is a rare tumour, only two cases of which have been reported so far. A third case arising in a 22 year old woman, who presented with abdominal pain and enlargement of the liver, is described. Ultrasound and a computed tomography scan showed a solitary hepatic mass, 12 cm diameter, involving both lobes of the liver. Serum immunoelectrophoresis revealed an IgG kappa monoclonal gammopathy. Histologically, the tumour was composed of mature plasma cells with mild atypia. The plasma cells infiltrated the liver parenchyma and showed kappa light chain restriction. The monoclonal nature of the tumour was also demonstrated by PCR amplification of the immunoglobulin heavy chain genes. There was no evidence of bone involvement and repeated bone marrow aspirates and biopsy specimens were normal. The patient was treated with eight courses of chemotherapy. One year after diagnosis, the patient is well, the size of the tumour has decreased and the paraproteinaemia has disappeared.