Unidirectional valve malfunction causes re-breathing of expired gas during anesthesia. Capnography is a useful method for monitoring the integrity of anesthetic equipment such as the unidirectional valves in a circular system. We report, here a case in which the capnography did not sound any rebreathing alarm, but the capnogram showed a prolonged plateau, an apparently short inspiratory period and a gradually sloping descending limb that stopped just short of baseline. As a result, capnography helped to identify an inspiratory unidirectional valve malfunction during the course of anesthesia. This study emphasizes the need to analyze the capnogram during every procedure involving anesthesia, in addition to preoperative checking of the Unidirectional valve.