Background Anemia is present in up to two-thirds of patients undergoing colorectal surgery mainly caused by iron deficiency and inflammation. As anemia is associated with increased risk of perioperative death, diagnosis and treatment of preoperative anemia according to etiology have been recommended. Objective The aim of the present study was to assess if the association between anemia and survival in patients undergoing colorectal surgery was determined by the severity of anemia alone or also by anemia etiology. Methods To determine the prevalence of anemia and etiology, preoperative hematological parameters, C-reactive protein, ferritin and transferrin saturation were retrospectively assessed and correlated with outcome in a cohort of patients undergoing colorectal surgery between 2005 and 2019 at the University Hospital of Innsbruck. Anemia was defined as hemoglobin <120 g/L in females and <130 g/L in males. The etiology of anemia was classified on the basis of serum iron parameters, as iron deficiency anemia, anemia of inflammation or other anemia etiologies. Results Preoperative anemia was present in 54% (1316/2458) of all patients. Anemia was associated with iron deficiency in 45% (134/299) and classified as anemia of inflammation in 32% (97/299) of patients with available serum iron parameters. The etiology of anemia was a strong and independent predictor of survival, where iron deficiency and anemia of inflammation were associated with better postoperative survival than other anemia etiologies. One year survival rates were 84.3%, 77.3% and 69.1% for patients with iron deficiency anemia, anemia of inflammation and other anemia types. Inflammation indicated by high C-reactive protein is a strong negative predictor of overall survival. Conclusions Anemia has a high prevalence among patients undergoing colorectal surgery and rational treatment requires early assessment of serum iron parameters and C-reactive protein.