In view of recent enthusiasm for surgery to treat morbid obesity, we examined national changes in utilization and in-hospital outcomes of bariatric surgery over time. With the use of International Classification of Diseases (ICD-9) codes, we identified all bariatric procedures (n=12,203) performed on adults from 1990 to 1997 in hospitals participating in the Nationwide Inpatient Sample. We then applied sampling weights and United States Census data to calculate the national population-based rates of bariatric surgery procedures for each year and examined secular trends in utilization. We further evaluated changes in patient characteristics and in-hospital mortality and complications. From 1990 to 1997, the national annual rate of bariatric surgery increased from 2.7 to 6.3 per 100,000 adults (P<0.001). The percentage of bariatric procedures performed by gastric bypass increased from 52% to 84% (P<0.001). Patients were slightly older (38.1 years vs. 40.3 years;P<0.001) with more comorbid conditions (20.9% vs. 31.6%; P<0.001) in 1997 vs. 1990. In-hospital mortality was 0.37% overall and remained stable. Rates of pulmonary emboli, early reoperation, and pulmonary complications declined significantly over time. Between 1990 and 1997, the annual rate of bariatric surgery in the United States more than doubled, without substantial changes in perioperative morbidity or mortality. This trend was largely associated with an increase in the use of gastric bypass procedures.