Pyrolysis of tissue in a hypoxic environment can produce carbon monoxide. The atmosphere of the peritoneal cavity is rendered hypoxic during laparoscopic cholecystectomy by insufflation with 100% carbon dioxide. To determine whether carbon monoxide is produced by electrocautery of tissue during laparoscopic cholecystectomy, nine patients undergoing this procedure had the insufflation gas after use of electrocautery analyzed for carbon monoxide. Blood was analyzed for carboxyhemoglobin in these same patients to determine whether carbon monoxide was being absorbed in dangerous amounts. Carbon monoxide was present in the peritoneal cavity 5 min after use of electrocautery was initiated at a median concentration of 345 ppm (range 25- 1600 ppm), and at the end of surgery at a concentration of 475 ppm (range 100-1900 ppm). This was well in excess of the 35 ppm upper limit for a 1-h exposure set by the Environmental Protection Agency. The carboxyhemoglobin concentrations (mean ± SD) were the same at the beginning (1.3% ± 0.7%), end (1.2% ± 0.7%), and the day after surgery (1.1% ± 0.6%). Although there was no evidence of significant absorption of carbon monoxide in these patients, care should be taken to scavenge the gases produced by cautery of tissues to avoid operating room contamination during laparoscopic surgery.