Few data exist concerning preoperative nutritional status in patients undergoing bariatric surgery. We retrospectively analyzed the preoperative values of serum albumin, calcium, 25-OH vitamin D, iron, ferritin, hemoglobin, vitamin B₁₂, and thiamine in 379 consecutive patients (320 women and 59 men; mean body mass index 51.8±10.6 kg/m²; 25.8% white, 28.4% African American, 45.8% Hispanic) undergoing bariatric surgery between 2002 and 2004. Preoperative deficiencies were noted for iron (43.9%), ferritin (8.4%), hemoglobin (22%; women 19.1%, men 40.7%), thiamine (29%), and 25-OH vitamin D (68.1%). Low ferritin levels were more prevalent in females (9.9% vs. 0%; P=0.01); however, anemia was more prevalent in males (19.1% vs. 40.7%; P<0.005). Patients younger than 25 years were more likely to be anemic than patients over 60 years (46% vs. 15%; P<0.005). This correlated with iron deficiency, which was more prevalent in younger patients (79.2% vs. 41.7%; P<0.005). Whites (78.8%) and African Americans (70.4%) had a higher prevalence of vitamin D deficiency than Hispanics (56.4%), P=0.01. Whites were the least likely group to be thiamine deficient (6.8% vs 31.0% African Americans and 47.2% Hispanics; P<0.005). Nutritional deficiencies are common in patients undergoing Roux-en-Y gastric bypass, and these deficiencies should be detected and corrected early to avoid postoperative complications.