Mantle cell lymphoma (MCL) has a characteristic chromosomal translocation, t(11:14) (q13;q32) involving rearrangement of bcl-1 locus, and the key oncogene of bcl-1 locus in PRAD1/cyclin D1 gene that encodes the protein regarding cell cycle. Recently, several studies using immunohistochemical and molecular methods have demonstrated the overexpression of cyclin D1 mRNA/protein in cases of MCL. We have studied immunohistochemical expression of cyclin D1 protein on frozen sections of 27 cases of MCLs and evaluated the relationship between the expression of cyclin D1 and prognosis. Sixteen (59.3%) cases showed intranuclear staining of cyclin D1 protein and 6 of 7 cases examined using RT-PCR methods showed the overexpression of PRAD1/cyclin D1 mRNA. The data indicate that intranuclear staining of cyclin D1 protein is associated with the overexpression of PRAD1/cyclin D1 mRNA. The survival time of cyclin-D1 positive group was shorter than that of cyclin D1-negative group, and there was a significant difference in survival time between the two groups (p < 0.05; log-rank test). These data suggest that the MCLs with overexpression of PRAD1/cyclin D1 protein has poor prognosis, and intranuclear expression of cyclin D1 protein is a useful prognostic marker for MCL.