Patients undergoing breast cancer treatment are susceptible to changes in eating behavior and nutrition status, thus making nutrition education relevant. Based on the World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR) guide, with recommendations for cancer prevention released in 2007, the present study aimed to evaluate the effectiveness of educational nutritional intervention for women with breast cancer undergoing treatment while adhering to the 2007 WCRF/AICR guidelines. A nonrandomized clinical trial was conducted with a population composed of female patients, divided in intervention (IG, n = 18) and comparison (CG, n = 68) groups. Data were evaluated at baseline and after treatment/nutritional intervention. The IG participated in the intervention program for 12 mo by biweekly phone calls, personal meetings, and monthly handouts, while targeting the intake of at least 400 g/day of fruits and vegetables, and no more than 500 g/week of red or processed meats. The recommended adherence to physical activity and food intake was evaluated in compliance with the guidelines. The IG improved their adherence to the guidelines and there was a significant decrease in consumption of animal food (-43.9%) and alcoholic drinks (-0.2 g ethanol/d) and increase in plant food intake (+65.4%). Most women in both groups were sedentary and therefore violated the guidelines. The CG presented an increase in body mass index (+1.0 kg/m2) and waist circumference (+1.9 cm), while no change was observed in the IG. These findings are promising as nutritional intervention for women undergoing breast cancer treatment contributed positively to improving their adherence to the WCRF/AICR guidelines.