A 29-year-old man developed diffuse large B-cell lymphoma in a subpectoral pacemaker pocket that 6 years previously had been created in the chest for a titanium-covered pulse generator. The patient had an 8-cm-diameter dark red tumor with necrotic tissue on a keloidal surgical scar in the left side of the chest. Left axillary lymphadenopathy also was present. Laboratory studies showed an increased level of soluble interleukin 2 receptor and a normal level of lactose dehydrogenase. A biopsy specimen showed a diffuse large B-cell phenotype and monoclonal immunoglobulin H gene rearrangement. A gallium scintigraphy study showed abnormal accumulation in the left chest and left axilla. On the basis of these findings, we diagnosed diffuse large B-cell lymphoma, stage II. The patient received THP-COP chemotherapy (pirarubicin, cyclophosphamide, vincristine, and prednisolone) and radiotherapy, achieved complete remission, and was free of disease for 16 months after treatment. This case suggests that there was a relationship between the development of non-Hodgkin's lymphoma and the presence of chronic inflammation in the pulse generator pocket.