To further clarify risk factors for breast cancerin Japanese women, a self-administered questionnaire was completedby 157 cases with histologically confirmed breast cancerfrom 1989 to 1993 and by 369 ageand residential area matched controls in Gifu, Japan.Conditional logistic regression model was used to assessthe relations. Multivariate analyses showed that breast cancerrisk decreased with body mass index for premenopausalwomen (RR=0.45; 95% CI=0.22–0.92for BMI 23 vs. < 21="">²)),but the risk increased with body mass indexfor postmenopausal women (RR=1.98; 95% CI= 0.86–4.55 for BMI 24 vs. <21.5>²)). The risk increased with a smallnumber of births in pre- and post-menopausal women(1.83; 1.11–2.99 and 6.06; 2.40–15.3 for 1–2 birthsand nulliparity, respectively, vs. 3 births). Ex-or current smoking increased the risk of breastcancer (2.31; 1.19–4.49). Reduced risk of premenopausal breastcancer was associated with high energy expenditure inphysical activity during teenage, although the trend wasnot statistically significant.