Background : Epidemiological and experimental evidence suggests that breast cancer risk can be reduced by dietary measures. Study of the relationships between dietary habits and prognosis in patients with breast cancer is essential to the design of diet intervention trials. Purpose : Our purpose was to determine whether dietary habits are associated with disease-free survival in patients with breast cancer who have undergone treatment. Methods : We interviewed 240 women about their dietary histories. These women were 50–65 years old and had pathological stage I-II breast cancer with subsequent follow-up for 4 years; 209 of these women were postmenopausal. Differences in dietary variables between groups of patients were analyzed with bivariate and multivariate statistical methods. Results : Cancers were classified as estrogen receptor (ER) rich (≥0.10 fmol/μg of DNA) in 149 patients and as ER poor (Conclusions : Dietary habits at the time of diagnosis may affect prognosis, at least for patients with ER-rich breast cancers. Dietary fat may have an effect on growth or spread of breast cancer, both of which may vary according to type of fat. Total fat and saturated fatty acids were the dietary parameters most strongly associated with risk for treatment failure. Implications : Dietary intervention might serve as an adjuvant treatment to improve breast cancer prognosis. [J Natl Cancer Inst 85: 32–36, 1993]