BACKGROUND: Preoperative anemia has been associated with increased risk of allogeneic blood transfusion and postoperative morbidity and mortality. The prevalence of preoperative anemia and its association with postoperative outcomes has not previously been reported in relation to fast-track elective total hip arthroplasty (THA) and total knee arthroplasty (TKA). We aimed to evaluate the prevalence of preoperative anemia in elective fast-track THA and TKA and its association with risk of perioperative transfusion, prolonged length of hospital stay (LOS), and postoperative readmission. STUDY DESIGN AND METHODS: This was a prospective observational database study with data obtained from six high-volume Danish fast-track surgical centers. Preoperative hemoglobin and patient demographics were collected prospectively using questionnaires while outcome and transfusion data were collected using national databases and patient charts. Adjusted risk estimates for transfusion, prolonged LOS, and all-cause readmission according to preoperative anemia status were obtained by multivariate logistic regression. RESULTS: A total of 5.165 THA or TKA procedures were included with a mean patient age of 67 ± 11 years and a median LOS of 2 (interquartile range, 2-3) days. A total of 662 patients (12.8%) had preoperative anemia according to World Health Organization classification. Preoperative anemia was associated with increased risk of receiving transfusion during admission (odds ratio [OR], 4.7; 95% confidence interval [CI], 3.8-5.8), increased risk of readmission within 90 days from surgery (OR, 1.4; 95% CI, 1.1-1.9), and increased risk of LOS of more than 5 days (OR, 2.5; 95% CI, 1.9-3.4) after adjustment for preoperative patient-related risk factors. CONCLUSION: Preoperative anemia in elective fast-track THA and TKA is independently associated with transfusion and increased postoperative morbidity, supporting the need for preoperative evaluation and treatment.