OBJECTIVE A case-control-family study of breast cancer in women under the age of 40 was carried out in Melbourne and Sydney, Australia, from 1992 to 1995 to determine the risk factors for these women. Subjects included 467 incident cases identified by state cancer registries and 408 population-based controls. METHODS All participants completed a structured risk-factor questionnaire and family pedigree during an in-person interview. Where possible, cancers in first- and second-degree relatives were verified. RESULTS Multiple logistic regression analysis showed that the strongest risk factor for breast cancer was a family history of the disease -- having at least one affected first-degree relative trebled the risk (relative risk [RR] = 3.3, 95 percent confidence interval [CI] = 1.9-5.8). Risk increased with height by three percent (standard error [SE] of one percent) per cm, and after adjusting for height, there was evidence for a decreased risk in women weighing 73 kg or more. There was an increased risk of breast cancer after the first full-term birth (RR = 1.8, CI = 1.0-3.5) but this risk fell by 30 percent (SE = 11 percent) with each subsequent livebirth. CONCLUSIONS The effects of other reproductive factors and oral contraceptive use, although not nominally significant, were in accord with published findings from similar studies in young women. This study of Australian women has indicated that some risk factors for breast cancer in women under age 40 differ from those reported for older women either in direction (e.g., weight) or relative importance (e.g., family history).