A self-administered questionnaire was completed by 497 Chinese women in Beijing with histologically confirmed breast cancer and by an equal number of age- and neighborhood matched control women. High body weight (71 + kg) was a risk factor for breast cancer in women aged 50 + (RR = 1.90), but this effect did not quite reach statistical significance. Nulliparity and late age at first birth were associated with an elevated risk of breast cancer. In comparison to parous women who had their first child before age 20, those who delayed this event until after age 29 had an RR of 1.65. The comparable RR for nulliparous women was 3.72. Late age at menopause was a risk factor for breast cancer. The RR for menopause after age 50 compared to menopause before age 45 was 2.16. High parity and long duration of lactation were protective factors against breast cancer. These 2 factors were highly correlated with each other and also with age at first birth. The effect of each on breast cancer risk was somewhat reduced after adjustment for the other two. A personal history of benign breast disease (RR = 3.21) and a family history of breast cancer (RR = 2.17) were also associated with an increased risk of breast cancer.