Cyclin D1 overexpression as a result of t(11;14) is a specific marker for diagnosis of mantle cell lymphoma (MCL) and plays an important role in MCL pathogenesis. To set a highly reliable cutoff value that discriminates MCL from other B-cell lymphoproliferative disorders, we performed a retrospective study of cyclin D1 expression. We established cyclin D1 expression level in 116 frozen and formalin-fixed, paraffin-embedded primary tumors from patients diagnosed with a variety of B-cell lymphoproliferative disorders. We used real time quantitative reverse-transcription polymerase chain reaction to quantify levels of cyclin D1 mRNA. The range of cyclin D1 expression in MCLs exceeded the range found in other lymphomas and reactive lymph nodes by a considerable margin. Cyclin D1 overexpression was found in 60/61 MCLs and in none of the other lymphomas, except for 12/19 mucosa-associated lymphoid tissue lymphomas from the lungs and stomach, which also revealed cyclin D1 overexpression. As epithelial tissues are known to express cyclin D1, an admixture of non-neoplastic epithelial cells present in the extranodal specimens probably influenced the quantitative reverse-transcription polymerase chain reaction result. Quantitative cyclin D1 monitoring provides a diagnostic test and an approach for studying MCL pathogenesis and may be of clinical importance.