A 34-year-old woman was hospitalized for the investigation of a one-month history of intestinal disorders, gastric heaviness and transitory icteric episodes. Extensive clinical investigations suggested the diagnosis of gall bladder carcinoma or sclerosing cholangitis. At laparotomy, the proximal part of common bile duct was markedly thickened by a white, firm, fish-flesh like tumour extending in to the cystic duct, gall bladder wall and to the liver. Histological study showed a diffuse lymphoid proliferation of the common bile duct mainly composed of small cells mixed with scattered large atypical cells. Immunohistochemistry revealed that most of the small cells expressed T-cell markers with predominant CD 4 and alpha-beta T-cell receptors and without phenotypic gap, whereas large atypical cells showed monotypic B phenotype with co-expression of mu and delta heavy chains and light lambda chain restriction. No evidence of primary nodal lymphoma was found during extensive clinical, radiological, sonographic or scanographic examinations. Sequential chemotherapy (MACOP-B) was instituted and the patient was still alive 4 years after diagnosis. Morphological and immunohistochemistry findings fulfilled criteria for a primary high grade B-cell lymphoma (centroblastic type, Kiel classification) from common bile duct concealed by numerous small reactive T-cells, so called T-cell rich B-cell lymphoma, not previously described in this location.