To explore risk factors for female breast cancer, a hospital-based case-control study was conducted in Tokyo, from 1990 to 1991. Information on potential risk factors was obtained by a selfadministered questionnaire from 5, 084 out-patients. Of the patients, 300 incident breast cancer cases were recruited, and 900 age-matched controls were randomly selected. Following significant findings emerged : (a) A large number of livebirths was associated with a decreased risk in premenopausal women (relative risk (RR) for 3 or more births relative to none : 0.24; 95% confidence interval (CI): 0.08-0.65). (b) In premenopausal women, regular menstrual cycle increased the risk (2.50; 1.16-5.38). (c) Current smokers experienced an increased risk (1.63; 1.11-2.39). (d) Heavy weight was associated with a greater risk of postmenopausal breast cancer: RR for those weighing 70 kg or more relative to those weighing 50 kg or less being 4.82 (1.53-15.2). (e) The later the age at first livebirth, the higher the postmenopausal breast cancer risk (2.85; 1.16-6.99; 3.54; 1.03-12.2 for ages of 30-34 and 35 years and more, respectively). J Epidemiol, 1994; 4 : 65-71.