African-American women with breast cancer consistently show a shortened survival when compared with Caucasians with breast cancer, however it is not clear whether this is due to socioeconomic factors or to racial differences in tumor biology. Cyclin D1 overexpression has been demonstrated in 60-80% of female breast cancers, however these studies have not included race or ethnicity data. We examined the level of cyclin D1 protein expression in 139 cases of female breast cancer obtained from different ethnic populations. Using an immunoperoxidase-based technique and a polyclonal anti-cyclin D1 antibody, the rate of overexpression was 68%. Cyclin D1 overexpression tended to be more frequent in cases from non-Caucasian patients when compared with those from Caucasian patients (77% vs. 59%, p=0.051). Our findings suggest that non-Caucasian ethnicity may be important in predicting cyclin D1 overexpression. Cyclin D1 could therefore serve as a possible target in managing breast cancer in the African-American population.