Cyclin D1 is a mediator of cell-cycle control that is frequently overexpressed in primary ductal breast carcinomas, but its role is controversial. A polymorphism in the CCND1 gene, G870A, results in an aberrantly spliced protein (cyclin D1b) lacking the Thr-286 phosphorylation site necessary for nuclear export. Studies of murine fibroblasts have shown that although overexpression of canonical cyclin D1 (cyclin D1a) alone is not sufficient to drive malignant transformation, expression of nuclear cyclin D1b is oncogenic. Our objectives were to determine whether cyclin D1b is expressed in human breast carcinomas and to characterize the relationship of this protein to both cyclin D1a and clinical outcome in breast cancer patients. We performed a prospective cohort study of women with early-stage breast cancer and analyzed cyclin D1a and D1b expression in primary breast tumor sections. Expression was tested for correlation with other breast cancer prognostic factors and clinical outcome, including recurrence or death. A total of 118 patients were included in this analysis, with a median follow-up of 44 months. Cyclin D1b was expressed in 26% of tumors and cyclin D1a was overexpressed in 27%; co-expression occurred in 4%. Cyclin D1a and/or D1b expression were not significantly associated with estrogen or progesterone receptor negativity, Her2 overexpression, young age, lymph node positivity, high tumor grade, nor large tumor size. The risk of recurrence was higher in those co-expressing D1a and D1b compared to the expression of either alone (relative risk=5.3, 95% confidence interval 1.27 to 22.1, p=0.02). The hazard ratio for those with co-expression compared with those without was 6.05 (p=0.04). Expression of cyclin D1b occurs in primary human breast carcinomas and its coexpression with cyclin D1a may be a marker for increased recurrence risk, independently of other factors.