Depo-medroxyprogesterone acetate (DMPA) is an injectable contraceptive that contains the same progestin as the menopausal hormone therapy regimen found to increase breast cancer risk among postmenopausal women in the Women's Health Initiative clinical trial. However, few studies have evaluated the relationship between DMPA use and breast cancer risk. Here, we conducted a population-based case-control study among 1,028 women ages 20 to 44 years to assess the association between DMPA use and breast cancer risk. Detailed information on DMPA use and other relevant covariates was obtained through structured interviewer-administered in-person questionnaires, and unconditional logistic regression was used to evaluate associations between various aspects of DMPA use and breast cancer risk. We found that recent DMPA use for 12 months or longer was associated with a 2.2-fold [95% confidence interval (CI), 1.2-4.2] increased risk of invasive breast cancer. This risk did not vary appreciably by tumor stage, size, hormone receptor expression, or histologic subtype. Although breast cancer is rare among young women and the elevated risk of breast cancer associated with DMPA appears to dissipate after discontinuation of use, our findings emphasize the importance of identifying the potential risks associated with specific forms of contraceptives given the number of available alternatives.