Anemia is a risk factor for increased postoperative morbidity and mortality. International guidelines, therefore, recommend preoperative diagnostic work up and causal treatment of anemia. Iron therapy, however, is suspected to negatively affect disease progression in patients with cancer-associated anemia. The objective of our systematic review was to assess the efficacy and safety of perioperative diagnosis and causal therapy of anemia, and to determine the effect of iron supplement on disease progression of cancer.We systematically searched multiple electronic databases. Two persons independently reviewed abstracts and full-text articles. We rated the risk of bias using the Cochrane Risk of Bias Tool and assessed the quality of the evidence using GRADE (Grading of Recommendations Assessment, Development and Evaluation). Meta-Analyses were performed using the DerSimonian&Laird random effects method. Results indicate that preoperative therapy of anemia could reduce the need for blood transfusions (relative risk: 0,78; 95% confidence interval 0,61-1,02; number needed to treat: 6) For other patient-relevant outcomes the number of events were too small to detect clinically relevant differences. We could not find any evidence that iron supplements have an influence on the progression of tumors.