The primary objective of this study is to assess the relationship between DMPA use and breast cancer risk among young women in a large population-based study. The specific hypotheses to be tested are: (1) Is DMPA use associated with an increase in the incidence of breast cancer in premenopausal women age 20-44? / Does the duration and/or recency of DMPA use influence the magnitude of the association? (2) Do demographic, reproductive, or anthropometric characteristics, such as parity and body mass index, modify this association? (3) Does the association between DMPA and breast cancer risk vary by histologic type or by the expression of steroid receptors including ERa, ERb, and PR?