A population-based case-control study was conducted to investigate the contribution of alcohol consumption during the early adult years (ages 18-35 years) and later adult years (older than age 35) to breast cancer risk. Alcohol consumption histories were obtained by questionnaire from 277 breast cancer cases, 372 population controls, and 433 controls with cancer of sites other than breast. Alcohol exposure during both age periods was significantly greater for breast cancer cases, but risks, estimated by maximum likelihood odds ratios, were highest for alcohol consumption frequency during the early adult period. Age-adjusted odds ratios and 95% confidence intervals (CI) for breast cancer risk and early age drinking (10 versus 0 drinks/week) were 2.2 (95% CI = 1.34, 3.5), relative to the population controls and 2.0 (95% CI = 1.3, 3.1) relative to the cancer controls. Based on later-age drinking of ten versus zero drinks/week, odds ratios were 1.8 (95% CI = 1.3, 2.6) and 1.6 (95% CI = 1.2, 2.2) relative to the population and cancer controls, respectively. The risk estimates were not altered by introduction of the following covariates into the analyses: mother with breast cancer, family status, education, body mass index, smoking, supplemental hormone use, and diet.