Mantle cell lymphoma (MCL) commonly lacks expression of CD23. However, a significant minority of MCLs express CD23, as assessed by flow cytometric immunophenotyping (FCIP). The aims of our study were to investigate the expression of CD23 by FCIP in patients with MCL and to correlate CD23 expression with pathologic and clinical parameters, including outcome. We studied 53 patients with untreated MCL who had CD23 expression determined by FCIP. At diagnosis, 14 MCLs (26%) were CD23+ at all tissue sites, whereas 33 (62%) were CD23-, and 6 (11%) had discordant CD23 expression among different tissue sites. Patients with CD23- MCL had extranodal disease more commonly compared with patients with CD23+ MCL. Moreover, with 57-month median follow-up, the 4-year event-free and overall survival rates for CD23+ MCL were 45% and 75%, respectively, compared with 19% and 51% for CD23- MCL. In multivariate Cox regression analysis, CD23 status and leukemic-phase MCL were the most important factors predicting outcome.