During a diagnostic CO2-hysteroscopy in general anesthesia, a manifest gas embolism with a resulting drop of the endexpiratory CO2 partial pressure occurred upon insertion of the instrument. By ending the procedure and through appropriate anesthesiological measures, the occurrence was brought under control and the embolism had no clinical consequences. The incidence encouraged us to reconsider the CO2-hysteroscopy examination technique. As a result, we describe an up to now neglected mechanism which may lead to air embolism in gas hysteroscopy: Similar to hysteroscopy with fluid distension, the whole system has to be purged from air by insufflating CO2 prior to examination. If this step is neglected, up to 40 cm3 of room air may be insufflated into the patient, considering a connective tubing of 200 cm length and 0.5 cm lumen. The scientific organisations as well as the endoscopic training centers and the manufacturers of hysteroflators are challenged to deal with this newly described potential cause of complications.