BACKGROUND. To assess the impact of T-cell/B-cell phenotype on clinical outcome, the authors retrospectively compared patients who had peripheral T-cell lymphoma, unspecified (PTCL-U), with patients who had diffuse large B-cell lymphoma (DLBCL). METHODS. Two hundred ninety-seven cases of PTCL-U and 496 cases of DLBCL that had been transferred from the files of the Intergruppo Italiano Linfomi or the Gruppo Italiano Linfomi were integrated into a unique working file and reviewed by the authors. RESULTS. The PTCL-U group and the DLBCL group had significantly different distribution patterns with respect to patient age, gender, disease stage, performance status (PS), the presence or absence of systemic “B” symptoms, the presence or absence of bulky disease, lactic acid dehydrogenase (LDH) levels, and number of extranodal sites (ENS). A significantly greater number of patients in the DLBCL group experienced complete remission (P 0.0001). Multinomial logistic regression analysis confirmed that immunophenotype, PS, LDH concentration, and number of ENS were independent predictors of response. At a median follow up duration of 43 months, there was no observable difference in disease-free survival (DFS) between patients with DLBCL and patients with PTCL-U; however, multivariate analysis did reveal that poorer PS and bone marrow involvement were significantly associated with shorter DFS. Furthermore, although the overall survival (OS) curves associated with the T-cell and B-cell immunophenotypes were significantly different from each other at a median follow-up duration of 37 months (P 0.0012), Cox multivariate analysis excluded immunophenotype from the final OS model.