Old age is associated with comorbidity and decreased functioning which influences treatment decisions in elderly breast cancer patients. The purpose of this study was to identify risk factors for complications after breast cancer surgery in elderly patients, and to assess mortality in patients with postoperative complications. The FOCUS cohort is a detailed retrospective cohort of all breast cancer patients aged 65 years and older who were diagnosed between 1997 and 2004 in the South-West of the Netherlands. Risk factors for postoperative complications were assessed using univariable and multivariable logistic regression models. One-year survival and overall survival were calculated using univariable and multivariable Cox Regression models, and relative survival was calculated according to the Ederer II method. 3179 patients received surgery, of whom 19 % (n = 618) developed 1 or more postoperative complication(s). The odds ratio of having postoperative complications increased with age [OR 1.85 (95 % confidence interval (CI) 1.37-2.50, p = 0.001) in patients >85 years] and number of concomitant diseases [OR 1.71 (95 % CI 1.30-2.24, p ≤ 0.001) for 4 or more concomitant diseases]. One-year overall survival, overall survival, and relative survival were worse in patients with postoperative complications [multivariable HR 1.49 (95 % CI 1.05-2.11), p = 0.025. HR 1.21, (95 % CI 1.07-1.36), p = 0.002 and RER 1.19 (95 % CI 1.05-1.34), p = 0.006 respectively]. Stratified for comorbidity, relative survival was lower in patients without comorbidity only. Increasing number of concomitant disease increased the risk of postoperative complications. Although elderly patients with comorbidity did have a higher risk of postoperative complications, relative mortality was not higher in this group. This suggests that postoperative complications in itself did not lead to higher relative mortality, but that the high relative mortality was most likely due to geriatric parameters such as comorbidity or poor physical function.