In the Working Formulation (WF), non-Hodgkin's lymphomas (NHL) are grouped according to their clinical behavior. These disorders are listed as entities defined by morphology, phenotype, and cytogenetics in the proposed Revised European-American Classification of Lymphoid Neoplasms (REAL), the clinical relevance of which is still debated. We analyzed 670 NHL cases included in two randomized clinical trials (EORTC 20855 WF-intermediate/high-grade and 20856 WF-low-grade malignancy) with histologic material available for review. Based on hematoxylin-eosin-stained sections, 77% of cases could be subtyped. Immunophenotyping was considered to be mandatory only in diagnosing T-cell lymphoma and anaplastic large-cell lymphoma. Of 522 cases subtyped, 11% were mantle cell lymphoma (MCL), 5% were marginal zone B-cell lymphoma (MZBCL), 46% were follicle center lymphoma, and 32% were diffuse large B-cell lymphoma. Statistical analysis and comparisons between classifications were made only within each trial and treatment group. MCL and MZBCL were characterized by a shorter median survival (3.4 and 4.1 years, respectively) in comparison with low- and intermediate-grade WF groups (> 9.3 and 5.8 years, respectively). In terms of progression-free survival, MCL showed a behavior similar to the low-grade group, with frequent relapses. Follicle center cell lymphomas behaved as low-grade lymphomas as defined by the WF and diffuse large B-cell lymphomas as the WF-intermediate grade group. Because several NHL entities have a clinical behavior of their own, their recognition by the REAL classification offers clinicians additional information that is not obtained when the WF is used.