The accuracy of 21 anaesthetists in estimating tidal volumes from reservoir bag movements was assessed using a model lung apparatus. The breathing system configuration (Mapleson A or D), the grade of anaesthetist, and the years of anaesthetic experience had no effect on accuracy. Greater precision of tidal volume estimation was observed with larger tidal volumes and lower fresh gas flows. The mean systematic error of 18 of the 21 anaesthetists was greater than zero, indicating a general tendency to overestimate tidal volume. This study therefore strengthens the view that clinical observations should be supplemented with information from continuous monitoring devices.