Loss of consciousness suddenly occurred in 2 healthy and nonsmoking patients undergoing bipolar resection for intramural myomas under spinal anesthesia. One patient had nystagmus and emesis. In both cases, neurological symptoms were of short duration. All other usual causes were excluded. Initial venous carboxyhemoglobin concentrations were 7% and 23%, respectively. These concentrations decreased within several hours with oxygen therapy. Carbon monoxide (CO) is a product of diathermic vaporization and may enter the blood and cause intraoperative cardiovascular symptoms. These are the first cases with neurological presentation. The CO poisoning was probably diagnosed early because the patients were awake.