The t(11;14)(q13;q32) chromosomal translocation, the hallmark of mantle cell lymphoma (MCL), is recurrently found in multiple myelomas (MM) by means of conventional cytogenetics. Unlike MCL, recent molecular studies of MM-derived cell lines with t(11;14) have indicated that the breakpoints are highly dispersed over the 11q13 region; however, the fact that cyclin D1 is generally overexpressed in these cell lines suggests that this gene is the target of the translocation. To evaluate further the involvement of cyclin D1 in MM, we used immunohistochemistry and fluorescence in situ hybridization to investigate cyclin D1 expression and the presence of chromosome 11 abnormalities in a representative panel of 48 MM patients (40 at diagnosis and 8 at relapse). Cyclin D1 overexpression occurred in 12/48 (25%) of cases; combined immunohistochemistry and fluorescence in situ hybridization analyses in 39 patients showed cyclin D1 positivity in all of the cases (7/7) bearing the t(11;14), in two of the 13 cases with trisomy 11, and in one of the 19 cases with no apparent abnormalities of chromosome 11. Our data indicate that the t(11;14) translocation in MM leads to cyclin D1 overexpression and that immunohistochemical analysis may represent a reliable means of identifying this lesion in MM.