The possible association between cigarette smoking, alcohol consumption and the risk of development of breast cancer before the age of 45 was investigated by means of a population-based case-control study in Sweden and Norway. Information was obtained, by personal interview, from 422 (89.2%) of all eligible patients with breast cancer newly diagnosed between May 1984 and May 1985, and from 527 (80.6%) of all age-matched controls. The possible confounding effects of oral contraceptive (OC) use, education, and reproductive and several other factors were taken into account in multivariate analyses. No association was found between ever smoking (versus never smoking) and breast cancer (odds ratio 1.0; 95% confidence interval (CI) 0.8-1.3). Further, there was no relation between breast cancer and duration of smoking, age at start of regular smoking, length of time since the start of regular smoking, or number of cigarettes smoked per day. There was no significant interaction between smoking, use of OCs, parity, and breast cancer. A moderate or high current consumption of beer, wine, liquor or total alcohol did not increase the risk of breast cancer. An alcohol intake of 5 grams per day or more was associated with a decreased risk of breast cancer (odds ratio 0.6; 95% CI 0.4-0.9), but possible effects of a change in habits after diagnosis, of recall bias and of residual confounding, e.g. by dietary habits, need serious consideration.