Local tissue reaction was examined after subcutaneous implantation for 8 wk of small pieces of Marlex (polypropylene) and Dacron (polyester) in 11 extremely obese patients. Dacron gave a stronger chronic inflammatory reaction in 7 out of 11 patients, and a stronger foreign body giant cell reaction in all of them: Marlex is therefore perhaps more suitable than Dacron as a reinforcement band in gastric surgery for obesity. At reoperations due to complications after gastric banding the tissue reaction in the omentum which covered the band, was examined in 9 patients. No major difference in tissue reaction in the omentum compared to that in subcutaneous fat was found. None of the reoperations was caused by too strong a tissue reaction to the reinforcement band i.e. Marlex in 8 patients and Gore-tex (Teflon) in one patient.