Here we present a 40-yr-old male patient with an aggressive B-cell lymphoma, who presented 2 yr earlier with polyarthritis, and was responsive to steroids and oral methotrexate. Thereafter he developed skin and lung lesions which on biopsy consisted of mixed 'inflammatory' infiltrates with granulomatous vasculitis. A diagnosis of seronegative Wegener's granulomatosis was made and the patient received a combination of prednisone and cyclophosphamide with clinical improvement and clearance of the radiological lesions in the lungs. The patient was now completely asymptomatic for 1 yr, but then generalized lymphadenopathy appeared, which was shown by histopathology to be large B-cell lymphoma, also involving the bone marrow. Despite intensive chemotherapy, his disease could not be controlled because of primary chemoresistance, which was perhaps in some way related to exposure to the suboptimal doses of chemotherapy given during the 'inflammatory' period before the diagnosis of lymphoma was established. This case illustrates the occasional difficulty in distinguishing between extranodal lymphoproliferative diseases and autoimmune disorders especially when clonality cannot be proved. It also shows the possible risk of 'masking' a true lymphoma by treating non-malignant diseases with immunosuppressive agents, which may eventually contribute to the development of chemoresistant lymphoma.