BACKGROUND: A prospective study was conducted to determine whether patients with larger breasts show larger tumors. MATERIALS AND METHODS: 123 patients with primary breast cancer were evaluated (median follow-up 36 months). Breast size was determined as a reflection of cup size. RESULTS: Cup size correlated with tumor diameter at diagnosis (p = 0.0038). On average, cup sizes were larger in postmenopausal than in premenopausal women (p = 0.0011), however, postmenopausal women did not show larger tumors than premenopausal. There was a significant positive correlation between body mass index, body weight and breast size (p < 0.0001). The mode of tumor detection (self vs. not self) was independent of breast size. No correlation was found between breast size and the patient's height, duration of breast feeding, smoking behavior or amount of alcohol consumption, hormone replacement therapy or use of oral contraceptives, number of involved lymph nodes or DFS and OS. CONCLUSION: Patients with larger breasts showed larger tumors. The mode of tumor detection is independent of breast size.