PURPOSE: To describe the relationship of body mass index (BMI) with postoperative complications and resource utilization. PATIENTS AND METHODS: Two thousand nine hundred and sixty-four patients 50 years or older undergoing elective noncardiac surgery with an expected length of stay > or = 2 days were enrolled in a prospective cohort study to measure major cardiac complications, noncardiac complications, length of stay, and costs. The setting was an urban teaching hospital. A preoperative history, physical, electrocardiogram (ECG), and chart review were performed by study personnel. Postoperative complications were detected by ECGs, creatine kinase and creatine kinase MB levels, and daily chart review. Total costs were obtained from the hospital's computerized database. RESULTS: Complication rates were not different among BMI groups (underweight < 20, normal 20 to 29, overweight 30 to 34, most overweight > 34), but patients with BMI 30 to 34 and > 34 who underwent abdominal or gynecologic procedures had significantly higher wound infection rates (11% each) than normal weight patients (4.7%) or the underweight (0%). After adjusting for age, race, gender, smoking history, comorbid diseases, procedure type, and insurance status, there were nonsignificant trends toward increased resource utilization by the most overweight patients (BMI > 34). These patients stayed 0.8 days longer (P = 0.13) and had total costs that were $843 higher (P = 0.17) than patients of normal weight (BMI 20 to 29). The underweight patients stayed 0.9 days longer (P = 0.23) and had total costs that were $3,150 higher (P = 0.04) than patients of normal weight. Quadratic models to test for a U-shaped relationship found no correlation between BMI and length of stay, but did find that BMI was significantly correlated with total costs (P = 0.04). This relationship persisted when patients who had complications were excluded from the analysis. CONCLUSIONS: Overall, BMI was not significantly correlated with postoperative complications or length of stay. However, overweight patients who underwent abdominal or gynecologic procedures had higher wound infection rates, and patients with the highest and lowest BMIs had significantly higher adjusted total costs.