Abstract: Epidemiologic studies have provided information on risk factors for breast cancer. Gail and associates identified five risk factors using the Breast Cancer Detection Demonstration Project (BCDDP) population and developed a model to calculate a composite relative risk (RR). This model is commonly used to counsel women regarding their risk for breast cancer and was used by the National Surgical Adjuvant Breast Project (NSABP) for eligibility for the Breast Cancer Prevention Trial. Because the BCDDP population was composed almost entirely of women 40 years of age or older, our purpose was to evaluate the effectiveness of the Gail model in estimating the risk of breast cancer for women under 40 in the clinical setting. The Gail risk factors were assessed for 124 patients under the age of 40 treated for either ductal carcinoma in situ (DCIS) or invasive breast cancer at the Lahey Hitchcock Medical Center between 1983 and 1995. The RR was calculated using the Gail model. For comparison, two cohorts of women under the age of 40 were used: 107 randomly selected patients who underwent a breast biopsy because of a benign condition and 129 nurses from our institution who responded to a questionnaire that included reproductive and family history information as used in the Gail model. The RR calculated was the RR that existed at the time of the surgical consultation for a suspicious breast lesion. The Tarone-Ware method was used to analyze statistical significance of differences between distribution. Contingency tables were analyzed using Miettinen's modification of Fisher's exact test. No differences were found between the median RR for all groups. Only 2 of the 124 patients with breast cancer had a RR of 5 or more (the RR required to enter the Breast Cancer Prevention Trial). The distribution of age at menarche (AGEMEN) was the same for each group. No difference was found for the distribution of age at first live birth (AGEFLB) between those with breast cancer and those with a benign biopsy or the control group. The number of breast biopsies (NBIOPS) was higher in patients with a benign breast biopsy. No difference was found in the distribution of number of first-degree relatives with breast cancer (NUMREL). Overall the Gail model failed to differentiate those women about to have cancer diagnosed from two control populations. The Gail model is not useful in identifying immediate risk of breast cancer in women under 40 and should not be used for that purpose.