Animal and in vitro studies suggest that the use of aspirin may be associated with reduced risk for breast cancer, but results from these studies of the association have been inconsistent. The objective of this meta-analysis was to quantitatively summarize the current evidence for such a relationship. We searched MEDLINE for studies of aspirin use and breast cancer risk that were published in any language, from January 1, 1966, to July 1, 2011. A total of 33 studies (19 cohort studies, 13 case-control studies, and 1 randomized controlled trial [RCT]) that included 1,916,448 subjects were identified. We pooled the relative risks from individual studies using a random-effects model, heterogeneity, and publication bias analyses. In a pooled analysis of all studies, aspirin use was associated with reduced risk for breast cancer (odds ratio [OR] = 0.86, 95% confidence interval [CI] = 0.81, 0.92). In the subgroup analysis by study design, results were similar except for RCT (OR = 0.98, 95% CI = 0.87, 1.09). In conclusion, this meta-analysis indicated that regular use of aspirin may be associated with reduced risk of breast cancer. More RCT were needed to confirm this association in the future.