Pancoast's syndrome due to malignant lymphoma is extremely rare. A case of diffuse large B-cell lymphoma presenting as Pancoast's syndrome is described. A 66-year-old man complained of pain and weakness of the right arm, and CXR revealed a right apical lung tumour. Histological findings were consistent with it being a diffuse large cell type lymphoma and Southern blot analysis revealed clonal rearrangement of the immunoglobulin heavy-chain JH. Thus, the tumour in this patient was diagnosed to be diffuse large B-cell lymphoma. Malignant lymphoma is an extremely rare cause of Pancoast's syndrome and only five cases have been described. This is the first reported case of Pancoast's syndrome caused by B-cell lymphoma, which was accurately diagnosed by analysis of gene rearrangement.