After completing chemotherapy and achieving a complete remission (CR), patients with diffuse intermediate-grade lymphoma and immunoblastic lymphoma are usually considered cured if they are able to maintain that remission continuously for 24 months. Recently, we observed a number of patients with these disorders who relapsed after a continuous CR of greater than or equal to 30 months from the beginning of therapy or 24 months from completing chemotherapy. This finding led us to examine 503 consecutive cases to determine the risk of late relapse and their clinical and biologic features. We found that the overall risk of late relapse of those who attained CR was 6.8%, but several features at presentation were associated with a high risk: (1) the presence of a divergent histology; (2) a sclerosing large cell lymphoma; (3) a diagnosis based on an extranodal site with no nodal tissue available for examination. When none of these features were present, the risk of late relapse was minimal (only 3%). When any of these features was present, the risk was 14%. Most striking was the 43% late relapse rate of patients with divergent histology. All but one of the eight B-cell tumors studied at relapse showed kappa light chain restriction. Five of these eight has a low S phase at the time of relapse, suggesting chemotherapeutic selection of a clone of cells with a low proliferative potential that could have given rise to the late relapse. Nucleic acid flow cytometry and immunophenotypic studies on three tumors at initial diagnosis and after relapse failed to support the hypothesis of a second de novo lymphoma and were consistent with a true recurrence of the original tumor. The results of salvage chemotherapy in this group of late relapses showed a high CR rate (57%) but no evidence of a trend for cure in the time to treatment failure curve. In contrast to the experience with Hodgkin's disease, retreatment with the same or a similar regimen used for the original induction was not associated with durable response. Clinicians should be aware of the potential for a late relapse in cases with divergent histology and the need for new treatment strategies for such cases.