Adjuvant chemotherapy significantly decreases recurrences and improves survival in women with early breast cancer (BC). However, the side effects of chemotherapy include weight gain, which is associated with poorer prognosis. We have previously demonstrated that by means of a comprehensive dietary modification which aims at lowering insulin levels it is possible to reduce body weight and decrease the bioavailability of insulin, sex hormones and the growth factors linked to BC risk and prognosis. We are now going to present a randomized controlled study of adjuvant diet in BC patients undergoing chemotherapy. The diet was designed to prevent weight gain during chemotherapy treatment. Women of any age, operated for invasive BC, scheduled for adjuvant chemotherapy and without evidence of distant metastases, were randomized into a dietary intervention group and a control group. The intervention implied changing their usual diet for the whole duration of chemotherapy, following cooking classes and having lunch or dinner at the study centre at least twice per week. 96 BC patients were included in the study. The women in the intervention group showed a significant reduction in their body weight (2.9 kg on average), compared with the controls. They also significantly reduced body fat mass, waist and hip circumferences, biceps, underscapular and suprailiac skinfolds, compared with the women in the control group. Our results support the hypothesis that dietary intervention during adjuvant chemotherapy for BC is feasible and may prevent weight gain.