The purpose of this study was to examine the long-term outcome of Stage I large-cell non-Hodgkin's lymphoma patients treated with primary radiotherapy and to assess the potential influence of these results on the development of future treatment protocols for non-Hodgkin's lymphoma. Between April 1970 and July 1983, 15 pathological Stage I and four pathological Stage II large cell lymphoma patients were treated with primary radiotherapy at the University of Minnesota Hospital and Clinics. This paper focuses on the long-term outcome of the 15 Stage I patients. As of March 1995, with a median survival of 171 months, five of the 15 Stage I patients are alive: four with no sign of lymphoma and one with recurrence and a secondary malignant neoplasm (SMN). Estimated 5- and 10- year recurrence-free survival rates are 86% and 78%, respectively. Overall survival at 5 years is 80% and at 10 years, 67%. Six patients developed SMNs, all of which were solid tumors. Ten of the 15 patients died: four from SMNs, three from non-Hodgkin's lymphoma, and three from other causes without recurrence (two died of cardiovascular disease and one of a pulmonary embolism). Deaths due to lymphoma in pathologic Stage I patients treated by radiotherapy alone have not changed significantly since our last report in 1985; however, deaths due to SMNs have increased. To provide optimal treatment for early non-Hodgkin's large-cell lymphoma, examination of the long-term treatment risks provides useful information for newer treatment regimens that do not yet have long-term treatment outcomes available.