Mantle cell lymphoma (MCL) is a distinct clinicopathologic entity of non-Hodgkin's lymphoma, characterized by a monotonous proliferation of small to medium-sized lymphocytes with co-expression of CD5 and CD20, an aggressive and incurable clinical course, and frequent t(11;14)(q13;q32) translocation. We examined 151 cases of lymphoma with MCL morphology from a viewpoint of cyclin D1 overexpression, which is now easily detectable by immunohistochemistry. 128 cases (85%) showed positive nuclear staining for cyclin D1, while the remaining 23 (15%) were negative. Except for cyclin D1 immunohistochemistry, current diagnostic methods, including morphological and phenotypical examinations, could not make this distinction. Although both the cyclin D1-positive and -negative groups were characterized by male predominance, advanced stages of the disease, frequent extranodal involvement, and low CD23 reactivity, the cyclin D1-positive group showed a higher age distribution (P =.04), larger cell size (P =.02), higher mitotic index (P =.01), more frequent gastrointestinal involvement (P =.05), higher international prognostic index score (P =.05), and lower p27(KIP1) expression (P <.0001). Of particular interest is that cyclin D1-positive MCL showed significantly worse survival than cyclin D1-negative lymphoma (5-year survival: 30% versus 86%, P =.0002), which was confirmed by multivariate analysis to be independent of other risk factors. These data suggest that cyclin D1-positive and -negative groups may represent different entities and that the former closely fits the characteristics of classical, typical MCL. We therefore propose that cyclin D1-positivity should be included as one of the standard criteria for MCL, and that innovative therapies for this incurable disease should be explored on the basis of the new criteria.