The authors report two cases of unexpected nitrogen accumulation in the circuit during low flow anaesthesia with a fresh gas flow of 600 ml/min (O2:N2O = 1.1). Though the presence in the anaesthesia circuit of nitrogen eliminated by the patient is a common feature of closed circuit and low flow techniques, the magnitude and the speed of increase of inert gas concentration (compared with data from previous experiences) were highly suspicious for an external source. This was readily identified as a "mini" leak (30 ml/min of N2) from the air flowmeter, although his valve was in fully closed position. The report depicts an uncommon cause of air entry in the anaesthesia circuit and confirms the need for monitoring gases and vapours when closed circuit and low flow techniques are employed.