A meta-analysis was performed to determine whether the scientific literature provides enough evidence that hormone replacement therapy after menopause increases the risk of breast cancer. Studies were located by MEDLINE, supplemented by a hand search of all the references in the articles located. The papers were graded as to quality. Those considered unbiased were combined using Woolf's method. Thirty-seven original studies were found: 23 case-control, 13 cohort, and one clinical trial. Overall, a small but statistically significant relative risk (RR) figure of 1.06 was calculated. Women who experienced natural menopause seemed to be at increased risk (RR = 1.13). A significant weighted RR was observed in current hormone replacement therapy users, especially in those who had natural menopause (RR = 1.63). A nonsignificant increasing trend was found between duration of hormone replacement therapy and breast cancer risk, although the opposite was seen when the association was analyzed by time since last use. These results imply that hormone replacement therapy could promote breast cancer.