Intravascular large B-cell lymphoma (IVLBCL) is pathologically distinct with a broad clinical spectrum and immunophenotypic heterogeneity. A series of 96 patients with IVLBCL (median age, 67 years; range, 41-85 years; 50 men) was reviewed. Anemia/thrombocytopenia (84%), hepatosplenomegaly (77%), B symptoms (76%), bone marrow involvement (75%), and hemophagocytosis (61%) were frequently observed. The International Prognostic Index score was high or high-intermediate in 92%. For 62 patients receiving anthracycline-based chemotherapies, median survival was 13 months. CD5, CD10, Bcl-6, MUM1, and Bcl-2 were positive in 38%, 13%, 26%, 95%, and 91% of tumors, respectively. All 59 CD10- IVLBCL cases examined were nongerminal center B-cell type because they lacked the Bcl-6+MUM1- immunophenotype. CD5 positivity was associated with a higher prevalence of marrow/blood involvement and thrombocytopenia and a lower frequency of neurologic abnormalities among patients with CD10-IVLBCL. Compared with 97 cases of de novo CD5+CD10-diffuse LBCL, 31 cases of CD5+CD10-IVLBCL exhibited higher frequencies of poor prognostic parameters, except age. Multivariate analysis in IVLBCL revealed that a lack of anthracycline-based chemotherapies (P<.001, hazard ratio [HR]: 9.256), age older than 60 years (P=.012, HR: 2.459), and thrombocytopenia less than 100x10(9)/L (P=.012, HR: 2.427) were independently unfavorable prognostic factors; CD5 positivity was not. Beyond immunophenotypic diversity, IVLBCL constitutes a unique group with aggressive behavior.