Hyper-estrogenism is more common in obese than in non-obese women. Consequently obesity has been shown to increase the risk of hormone department tumors. Some investigators have claimed that obesity at the time of primary treatment may be an independent prognostic factor for breast cancer, but this issue is still controversial. Therefore, we conducted a retrospective analysis to assess the influence of obesity at the time of primary treatment on disease-free survival (DFS). Obesity was defined as an excess of more than 25% of ideal weight according to Broca's index ([Height (cm) -100])-10%). The Cox-model was used for multivariate analysis. Mean follow-up was 61 (range 6-126) months. 295 (62.3%) patients were classified as of normal weight and 178 (37.6%) as obese. Mean excess of ideal weight was 8.9 kilograms (kg) in premenopausal and 13.9 kg in postmenopausal patients (non-parametric t-test p < 0.00001). Patients with tumor size < 20mm, 20-50mm and > 50mm had a means excess of the real weight of 10.6kg, 12.5kg and 16.1kg, respectively (non-parametric t-test p < 0.0001). Percentual excess of real weight compared to ideal weight was 22.4 [+/-21.2] kg in patients without recurrence and 21.5 [+/-21.9] in patients with recurrent disease (nonparametric t-test p = 0.7256). Univariate analysis revealed no significant association between obesity and the DFS. Multivariate analysis identified axillary lymph node involvement as the only statistically significant prognostic factor for disease-free survival (RR 1.55; 95%-confidence interval 1.02-2.36; p:0.0368). Because of the high correlations and node-status, tumor size and histological grading, the other factors failed to be prognostically relevant in this analysis. Obesity was not found to influence DFS of patients with primary breast cancer and is therefore unlikely to constitute an independent prognostic factor. It may, however, contribute to delayed diagnosis, since a significant proportion of obese patients were diagnosed with local advanced disease.