Greater body fatness was found to be a risk factor for postmenopausal breast cancer (BC); furthermore this factor appears to be associated with poor prognosis. These findings were tested by analyzing 1459 patients whose cases were recorded in the Basler Mammakarzinom-Datenbank (1990-2009) and the Swiss Health Surveys (1992-2007). Considering only postmenopausal women, no association between a rising body mass index (BMI) and BC development was observed. BMI was found to be a significant factor for tumor size (this applied to self-detected tumors and for lesions detected by radiological examinations) and had a positive correlation to advanced disease stage, unfavorable grading and a higher St. Galler risk score. In most therapy modes, patients with increasing BMI demonstrated a higher compliance and persistence to adjuvant therapies.