Risk factors for breast cancer (BC) detected in mammography screening were sought using a questionnaire among 31,927 women aged from 40 to 74 years who attended screening. Data from 204 women with screen-detected BC were compared with those of 612 controls who did not have BC in screening. Mothers of women with BC had more often BC than those of the controls (8% vs. 3%, odds ratio, OR, 3.18, 95% CI 1.59-6.35). Women with screen-detected BC were older at their first childbirth (25.7 years vs. 24.7 years, OR 1.61, 1.14-2.33), and younger at menarche (13.6 years vs. 13.8 years, OR 1.38, 1.00-1.91), but there was no significant difference in the body mass index, number of pregnancies, breast size, smoking, or in the use of contraceptive pills between the cases and the controls. In multivariate logistic regression analyses, late age at the first childbirth, BC in the mother, and early age at menarche were independent risk factors for screen-detected BC but they appear to be of limited value in targeting screening in the female population aged from 40 to 74 in order to improve its cost-effectiveness.