Laparoscopic gastric-bypass surgery (LGBS) is one of the surgical treatments for morbid obesity, which is performed under general anesthesia. TAP block is an analgesic strategy covering the dermatomes from T6 to L1 and consists of injecting local anesthetic in the TAP between the costal margin and the iliac crest, where the thoracolumbar nerves (T6-L1) are located. As far as the investigators know, this analgesic technique has never been studied in LGBS. The investigators objective is to determine whether an ultrasound (US)-guided TAP block provides improved analgesia during the first 24 hours after laparoscopic gastric-bypass surgery, compared to conventional therapy only.