Context. The most advanced and fastest growing form of bariatric surgery is laparoscopic gastric bypass. Very little is known about population-based 180-day laparoscopic bypass costs, complication rates, readmission rates, and post-operative care. Objective. To examine the 6-month costs and outcomes of laparoscopic vs. open bariatric bypass surgery using a national population-based sample. Design. We use the 1998-2003 Nationwide Inpatient Sample to examine national trends in the rate of laparoscopic bypass. To examine post-operative outcomes, we examine insurance claims for 2,384 bariatric bypass surgeries, at 308 hospitals, among a population of 5.6 million non-elderly people covered by large employers across 49 states in 2001 and 2002. Multivariate logit regression analysis is performed to risk-adjust outcomes. Main Outcome Measures. 180-day outcomes: 12 complications specific to bariatric surgery and 44 general post-operative conditions, readmission rates, ER rates, and expenditures following bariatric surgery. Results. Between 1998 and 2003, the national percentage of bariatric bypass surgeries that were laparoscopic grew from 1.5 to 17.1%. There was no significant difference in in-hospital mortality between laparoscopy and open surgery. With the 2001-2002 claims data, we find that of the patients having bypass surgery, men had 48% lower odds of having laparoscopy and that high bariatric volume hospitals were close to four times more likely to use laparoscopy. Laparoscopic bypass, compared with open bypass, had 34% lower odds of a complication during the initial surgical stay, 27% lower odds of a 30-day complication, but no statistically significant difference in 180-day complications. Laparoscopy had 49% higher odds of having the general 44 post-operative conditions, with 45% higher odds of a readmission and 54% higher odds of an ER visit. However, overall, laparoscopy resulted in a 23% lower number of hospital days and 9% lower 180-day expenditures. Conclusion. The laparoscopic cost-savings during the less invasive initial surgery stay outweigh the increase in post-discharge utilization. Further cost-savings will only emerge from laparoscopy only if its late post-operative complications are reduced. More cost-savings will also emerge as more physicians switch to the use of laparoscopy for bypass surgery.