Pyrolysis of tissue in a hypoxic environment can produce carbon monoxide. Peritoneal cavity is hypoxic during laparoscopic cholecystectomy by insufflation with 100% carbon dioxide, then, carbon monoxide is produced by electrocautery of tissue. To determine whether carbon monoxide was being absorbed in dangerous amounts to patients and operating room workers, blood was analyzed for carboxyhemoglobin in patients and their operators. Twenty-one patients undergoing this procedure, sampling the insufflation gas before, after use of electrocautery analyzed for carbon monoxide. Carbon monoxide was present in the peritoneal cavity, 5 min after use of electrocautery at a median concentration of 430 ppm(range 20~1000 ppm), and at the end of surgery at a median concentration of 174 ppm(range 10~720 ppm). This is well in excess of the 35 ppm upper limit for a 1-hr exposure set by the Environmental Protection Agency in U.S.A.. The patients carboxyhemoglobin concentrations(mean ± SD) at the beginning, at the end and 3 hrs after surgery were 0.51±0.30%, 0.41%±0.28%, and 0.45%±0.74%, respectively. The operator's carboxyhemoglobin concentrations before and at the end of surgery were 1.12%±1.09% and 1.03%±1.03%, respectively. Although there was no evidence of significant absorption of carbon monoxide in these patients and operators, care should be taken to scavenge the gases produced by electrocautery of tissues to avoid operating room contamination during laparoscopic cholecystectomy.