Prognostic factors for long-term survival of 312 patients with diffuse large cell or immunoblastic non-Hodgkin's lymphoma are presented based on analysis of the multiinstitution clinicopathologic study sponsored by the National Cancer Institute. At the time of analysis, 75% of the patients had died and the median follow-up for patients still alive was 11 years. The distribution of Ann Arbor stages was 21% stage I, 32% stage II, 17% stage III, and 30% stage IV. Factors of prognostic significance for survival included age, stage, histologic subtype, presence of B symptoms, size of the largest lesion, number of extra-lymphoid organs involved, and extent of lymphatic involvement. Recursive partitioning analysis suggested a prognostic classification system based on stage, age, size of the largest lesion, and presence of mediastinal involvement. Stage I patient less than 50 years of age had a 10 year survival rate of 65% compared to 36% for older stage I patients. Stage II patients less than 65 years old without bulky lesions or mediastinal involvement had a 10 year survival rate of 45% compared to 10% for the poorer risk stage II patients. Although statistically significant prognostic factors were identified for the stage III/IV patients, they were not strong discriminants of 5-10 year survival rate. Because of the correlation among potential prognostic factors, there is no uniquely best classification system. Reasons for discrepancies among reported prognostic factor analyses are discussed, and a prognostic grouping that synthesizes our results with those of others is proposed.