Many physicians administer involved field radiation therapy (RT) following brief chemotherapy for localized aggressive non-Hodgkin's lymphoma. Involved field irradiation usually implies treatment to the involved nodal regions with and without the contiguous lymphatic region, however, there is no agreements about its definition. Here we assess the appropriateness of RT irrespective of lymph node regions (localized field) following chemotherapy for patients with early stage diffuse large B-cell lymphoma. The localized field encompassed all original gross tumor volumes before chemotherapy with at least a 2- to 3-cm margin irrespective of lymphatic regions. We also evaluated the suitable radiation dose on the basis of response to chemotherapy. Twenty five eligible patients were treated with 3 cycles of chemotherapy (CHOP) followed by RT. All 25 patients had disease confined to Waldeyer's ring and/or cervical lymph nodes. Twenty two patients in complete response following chemotherapy received 30 Gy, and the remaining 3 in partial response received 40 Gy. With a median follow up of 42 months, both event free and overall survival rates at 2 years were 96.0%. There were no in-field recurrences, however, two patients experienced relapses. One developed central nervous system involvement and subsequently died of his disease. The other had mediastinal and submental lymph node relapse at 32 months, and is alive after salvage chemotherapy. Our study demonstrated that it should be possible to reduce treatment volume to less than the conventional involved field, and to limit the dose of RT in the range of 30-40 Gy.