To seek explanations for the geographic variation of breast cancer across the continental United States, we calculated the correlations between mortality rates for premenopausal and postmenopausal women and demographic data for the 3,056 U.S. counties. The northern predominance of this tumor was primarily among postmenopausal women, whereas mortality among premenopausal women was distributed almost uniformly across the country. Socioeconomic status (particularly income), German ethnicity, and colon cancer mortality were strong indicators of the rates for postmenopausal women, but only partly explained the northern excess and latitudinal gradient. In contrast, fertility patterns and ovarian cancer mortality were more closely linked to breast cancer among premenopausal women. The geographic peculiarities of this tumor in older women suggest extrinsic risk factors that remain to be identified, whereas the patterns for younger women point to the primary role of reproductive and genetic determinants.