In addition to rising type 2 diabetes and breast cancer incidence rates worldwide, diabetes may also increase breast cancer risk, and the association may vary by ethnicity. This review summarizes published data evaluating the association between diabetes and breast cancer in women of Asian, Hispanic and African American ancestry while considering a measure of obesity, body mass index (BMI). Published reports were identified through a search of PubMed and previous publications. Of 15 age-adjusted studies, 11 reported on Asian women from various countries, 3 on Hispanics and 1 on African Americans. The studies of Asian women described significant associations in 8 reports, with risk estimates of 1.5 to 8.4, but 3 were case-control studies and 6 did not adjust for BMI. The 3 case-control studies of Hispanic people included BMI, but only 1 detected a weak association between diabetes and breast cancer risk and was limited to postmenopausal women. The only study of African American women was a prospective cohort, and it showed no significant association between diabetes and breast cancer. In contrast to a 10% to 20% higher risk for breast cancer associated with diabetes reported for Caucasian women, there is little evidence for an association in Hispanics and African Americans. Although several studies of Asian women included in our review reported a higher risk for breast cancer with diabetes, methodologic shortcomings, such as lack of adjustment for obesity, use of a general population as controls, case-control design and small sample sizes, raise questions about the validity of the findings.