A gene expression signature of tumor proliferation rate in mantle cell lymphoma (MCL) is an overriding molecular predictor of the length of survival following diagnosis. Many strongly proliferative MCL tumors have exceptionally high cyclin D1 mRNA levels and preferentially express short cyclin D1 mRNA isoforms. We demonstrate here that these short mRNAs are cyclin D1a isoforms with truncated 3′UTRs, not alternatively spliced cyclin D1b mRNA isoforms. Among 15 MCL tumors with truncated cyclin D1 mRNAs, 7 had genomic deletions in the CCND1 3′UTR region. In 3 others, CCND1 contained point mutations that created premature polyadenylation signals, giving rise to 1.5-kb mRNAs lacking most of the 3′UTR. Both types of genomic alteration created transcripts lacking mRNA destabilization elements present in the wild-type cyclin D1a mRNA. Premature polyadenylation due to a 3′UTR mutation also was present in the Z-138 MCL cell line, which expressed both truncated and full-length cyclin D1a mRNAs. In these cells, the half-life of the short cyclin D1a mRNA was much longer than that of the full-length mRNA. We conclude that alterations of CCND1 3′UTR structure can significantly increase its oncogenic effect and worsen the clinical course of MCL patients.