Introduction 1 5 5 8 9 12 13 15 6 16 17 18 19 20 In an effort to clarify inconsistent results from previous studies, we evaluated established breast cancer risk factors in relation to breast density in a large population of women enrolled in a statewide mammography registry. Our intention was to determine whether characteristics associated with breast cancer risk were also related to breast density, a finding that would be consistent with the notion that density mediates breast cancer risk. We were particularly interested in assessing the influence of menarcheal age. Methods 21 22 The epidemiological data used in this analysis arose from three sources: a self-administered questionnaire completed by the patient, a patient intake form administered face-to-face by the radiologic technologist, and a standardized clinical assessment form completed by the radiologist. The questionnaire collected height, weight, place of birth, ethnicity, marital status, education, insurance coverage, reason for the current visit, past history of clinical breast examinations and mammography, age at menarche, parity, and age at first birth. The questionnaire also queried women regarding the date of their last menstrual period and history of gynecological surgery. This information was used to classify women as premenopausal (still having periods naturally) or postmenopausal (periods had stopped permanently) either naturally, because of chemotherapy/radiation, or surgery). The patient intake form obtained date of birth, family history of breast cancer (in the subject’s mother, sister, daughter, or other relative), personal history of breast cancer, history of breast procedures, type of exam conducted at current visit, examination outcomes, recommendation for further work-up or follow-up, and current use of postmenopausal hormone therapy (HT). The clinical assessment form obtained the type of exam conducted at current visit, breast density, examination outcomes and recommendation for further work-up or follow-up. All three forms are completed during the woman’s first NHMN mammography visit. Patient intake and clinical assessment forms are also completed at subsequent mammography visits, and the questionnaire is updated as possible. ® ® 23 To optimize temporal correspondence between the women’s characteristics and the classification of breast density, the statistical analyses were, when possible, based on the woman’s breast density on the date of the first recorded mammogram. When data for variables (other than HT use) were unavailable for the date of the mammogram, we searched forward in the NHMN records to retrieve replacement information corresponding to a subsequent mammography visit. Informative forward searches retrieved information from subsequent mammographic encounters occurring, on average, within 24 months of the index mammogram, and reduced missing values by 3–9%. 2 24 p p 1 Table 1 Distribution of women’s characteristics by breast density, dichotomized as dense versus not dense n Not dense Dense n % n % Age (years) 30–39 6,728 8 10,011 16 40–49 24,970 31 29,954 48 50–59 21,537 27 13,597 22 60–69 15,160 19 5,433 9 70–79 10,349 13 3,039 5 80–89 2,476 3 774 1 All women 81,220 56 62,798 44 Education