The objective of the study was to compare the results of open versus laparoscopic gastric bypass in the treatment of morbid obesity. Gastric bypass is one of the most commonly acknowledged surgical techniques for the management of morbid obesity. It is usually performed as an open surgery procedure, although now some groups perform it via the laparoscopic approach. Between June 1999 and January 2002 we conducted a randomized prospective study in 104 patients diagnosed with morbid obesity. The patients were divided into 2 groups: 1 group with gastric bypass via the open approach (OGBP) comprising 51 patients, and 1 group with gastric bypass via the laparoscopic approach (LGBP) comprising 53 patients. The parameters compared were as follows: operating time, intraoperative complications, early (30 days) postoperative complications, hospital stay, and short-term evolution of body mass index. Mean operating time was 186.4 minutes (125-290) in the LGBP group and 201.7 minutes (129-310) in the OGBP group (P < 0.05). Conversion to laparotomy was necessary in 8% of the LGBP patients. Early postoperative complications (30 days) occurred in 11% of the LGBP group compared with 24% of the OGBP group (P < 0.05). The differences observed between the 2 groups are the result of a high incidence of abdominal wall hernias in the OGBP group. Mean hospital stay was 5.2 days (1-13) in the LGBP group and 7.9 days (2-28) in the OGBP group (P < 0.05). Evolution of body mass index during a mean follow-up of 23 months was similar in both groups. LGBP is a good surgical technique for the management of morbid obesity and has clear advantages over OGBP, such as a reduction in abdominal wall complications and a shorter hospital stay. The midterm weight loss is similar with both techniques. One inconvenience is that LGBP has a more complex learning curve than other advanced laparoscopic techniques, which may be associated with an increase in postoperative complications.