We examined breast cancer incidence in a cohort of about 69,000 women who were members of a large prepaid health plan in Northern California and who answered detailed questions about alcohol consumption from 1979 through 1984 as part of a voluntary multiphasic health checkup. Among white, black, and Hispanic women with no prior cancer, breast cancer had developed in 303 by the end of 1984. In analysis controlling only for age there was a progressive increase in breast cancer incidence with each higher level of reported alcohol consumption. In multivariate analyses controlled for age, race, body mass, and smoking, the relative risk at 1-2 drinks per day was 1.5 [95% confidence interval (CI) 1.0-2.3], at 3-5 drinks per day it was 1.5 (95% CI 0.8-2.8), and at 6 or more drinks per day it was 3.3 (95% CI 1.2-9.3). Past drinkers tended to have been heavier drinkers than current drinkers and had a relative risk of 2.2 (95% CI 1.2-3.9). Study of wine, beer, and liquor use did not suggest that any particular alcoholic beverage was responsible. Significant associations with heavy alcohol consumption were strongest among white, and among postmenopausal women. This study adds support to the growing evidence that alcohol may be a risk factor for development of breast cancer.