Purpose: To evaluate the feasibility of integrating a program based on dietary fat intake reduction into adjuvant treatment strategies for postmenopausal women receiving therapy for early breast cancer. Patients and Methods: Two hundred ninety postmenopausal women with localized (stage I to IIIa) breast cancer receiving conventional systemic therapy provided informed consent and were randomized in a multicenter trial to either a dietary intervention group receiving a program of individualized instruction for reducing total fat intake or a dietary control group with minimal dietary counseling. Results: Significantly reduced (P < .001) fat intake (in terms of percent calories derived from fat) was observed in the intervention group versus the control group at 3 months (20.3% ± 2.4% v 31.5% ± 2.6%, mean ± SD, respectively) and maintained throughout 24 months of observation. The 50% reduction in daily fat-gram intake (from 66 ± 23 to 33 ± 14 g, P < .001) seen at 6 months was associated with reduced saturated fat, monounsaturated fat, polyunsaturated fat, and linoleic acid (P < .001). Significantly lower body weight was also seen in intervention compared with control patients at all observation periods, resulting in a 3.3-kg weight difference 18 months after randomization (P < .001). Conclusion: Substantial and sustained dietary fat reduction with associated weight change can be achieved at relatively low cost within the context of conventional multimodality clinical management of postmenopausal women with localized breast cancer. This result supports the feasibility of conducting a full-scale evaluation of the influence of dietary fat intake reduction on the clinical outcome of breast cancer patients.