PURPOSE: The impact of dietary counseling on body composition in early breast cancer patients (EBC) treated with aromatase inhibitors (AIs) is uncertain. The aim of this study was to assess the effects of a diet counseling program on weight, BMI, total and regional body composition in patients treated with AIs. METHODS: This observational study involved 194 EBC patients, of which 97 attended a 6-month personalized counseling program, based on Mediterranean diet principles (cohort A) and 97 did not (cohort B). Dual-energy X-ray absorptiometry (DXA) scan was used to measure the total and regional fat and lean body mass, before (baseline) and after at least 18 months of AI-therapy. RESULTS: Weight and BMI increased significantly, on the average, in cohort B, but not in cohort A. In the cohorts A and B, fat mass increased by 10 % and 7.7 % respectively, while lean mass decreased by 3.3 % and 2.6 % from before to after AI therapy, without statistically significant differences between them using the Mann-Whitney test. The changes in body composition were greater in premenopausal than in postmenopausal women at cancer diagnosis. The proportion of patients with sarcopenia, obesity and sarcopenic obesity increased from before to after AI therapy, similarly in both cohorts. CONCLUSIONS: Patients treated with AIs reported an increase in fat mass and a decrease in lean mass, and consequently an increase in sarcopenia and obesity, regardless of the participation in a dietary counseling program. A combined dietary counseling and physical exercise program may be necessary for preventing these unfavourable changes in these patients.