Diabetes mellitus has been associated with an increased risk of several types of cancers, but its relationship with breast cancer remains unclear. We conducted a meta-analysis of case–control and cohort studies to assess the evidence regarding the association between diabetes and risk of breast cancer. Studies were identified by searching MEDLINE (1966–February 2007) and the references of retrieved articles. We identified 20 studies (5 case–control and 15 cohort studies) that reported relative risk (RR) estimates (odds ratio, rate ratio/hazard ratio, or standardized incidence ratio) with 95% confidence intervals (CIs) for the relation between diabetes (largely Type II diabetes) and breast cancer incidence. Summary RRs were calculated using a random-effects model. Analysis of all 20 studies showed that women with (versus without) diabetes had a statistically significant 20% increased risk of breast cancer (RR, 1.20; 95% CI, 1.12–1.28). The summary estimates were similar for case–control studies (RR, 1.18; 95% CI, 1.05–1.32) and cohort studies (RR, 1.20; 95% CI, 1.11–1.30). Meta-analysis of 5 cohort studies on diabetes and mortality from breast cancer yielded a summary RR of 1.24 (95% CI, 0.95–1.62) for women with (versus without) diabetes. Findings from this meta-analysis indicate that diabetes is associated with an increased risk of breast cancer. © 2007 Wiley-Liss, Inc.