BACKGROUND: Current information on the prognostic importance of body mass index (BMI) for patients with early breast cancer is based on a variety of equivocal reports. Few have data on BMI in relationship to systemic treatment. PATIENTS AND METHODS: Patients (6792) were randomized to International Breast Cancer Study Group trials from 1978 to 1993, studying chemotherapy and endocrine therapy. BMI was evaluated with eight other factors: menopausal status, nodal status, estrogen receptor status, progesterone receptor status, tumor size, vessel invasion, tumor grade and treatment. BMI was categorized as normal (< or =24.9), intermediate (25.0-29.9) or obese (> or =30.0). RESULTS: Patients with normal BMI had significantly longer overall survival (OS) and disease-free survival (DFS) than patients with intermediate or obese BMI in pairwise comparisons adjusted for other factors. Subset analyses showed the same effect in pre- and perimenopausal patients and in those receiving chemotherapy alone. When assessed globally and adjusted for other factors, BMI significantly influenced OS (P = 0.03) but not DFS (P = 0.12). CONCLUSIONS: BMI is an independent prognostic factor for OS in patients with breast cancer, especially among pre-/perimenopausal patients treated with chemotherapy without endocrine therapy.