A relatively high number of fatal complications during hysteroscopy, where carbon dioxide was used as the uterine distension medium, plus a recent report on heart embolism during dog experiments with venous carbon dioxide infusion, audible by simple stethoscopic surveillance during the infusion, prompted the present study. Seventy women with no history of cardiac valvular disease or failure were observed during carbon dioxide hysteroscopy for the occurrence of the characteristic metallic heart sound caused by the intracardiac presence of free carbon dioxide during the heart contractions. In seven cases (10%) the typical metallic heart sounds appeared during the hysteroscopy, leading to immediate interruption of the hysteroscopy and withdrawal of the hysteroscope. This caused the pathological heart sounds to disappear, after which the hysteroscopy could continue. Heart auscultation should always be performed during carbon dioxide hysteroscopy in order to avoid serious cardiovascular complications. This simple precaution turns carbon dioxide hysteroscopy into a versatile and safe method for intra-uterine diagnosis and therapy.