A method and arrangement for detecting a leak in an anesthesia system. The method includes controlling respiratory movements by means of ventilator gas flows and measuring the ventilator gas flow added for an inspiration and removed for an expiration. The method also includes supplying a fresh gas flow for a respiration and measuring the fresh gas flow added for the respiration. The method further includes receiving information indicative of the measured gas flows, determining based on said received information both the gas volume added and the gas volume removed and comparing these determined gas volumes to each other. The method also includes determining based on comparing the anesthesia system leakage.
