Anemia is the most common nutritional deficiency after bariatric surgery. The predictors of anemia have not been clearly identified. This issue is useful for selecting an appropriate surgery procedure for morbid obesity. From December 2000 to October 2007, a retrospective study of 442 obese patients after bariatric surgery with two years' follow-up data was conducted. Anemia was defined by hemoglobin (Hb) under 13mg/dL in male and 11.5mg/dL in female. We analyzed the clinical information and laboratory data during the initial evaluation of patients referred to bariatric surgery for predictors of anemia development after surgery. All data were analyzed by using multivariate adaptive regression splines (MARS) method. Of the patients, the mean age was 30.8±8.6 years; mean BMI was 40.7±7.8kg/m2 and preoperative mean hemoglobin (Hb) was 13.7±1.5g/ dL. The prevalence of anemia increased from preoperatively 5.4% to 38.0% two years after surgery. Mean Hb was significantly lower in patients receiving gastric bypass than in restrictive type surgery (11.9mg/dL vs. 13.1mg/dL, p=0.040) two years after surgery. Besides, the preoperative optimal value of hemoglobin to predict future anemia in MARS model is 15.6mg/dL. The prevalence of anemia increased to 38.0% two years after bariatric surgery. We obtained an optimal preoperative value of hemoglobin 15.6mg/dL to predict postoperative anemia, which was important in preoperative assessment for bariatric surgery. Patients undergone gastric bypass surgery developed more severe anemia than gastric banding or sleeve gastrectomy.