Ultrasound-spirometry and capnography and the evaluation of corresponding 'single-breath diagrams for CO2' (SBD-CO2) is a relatively new, non-invasive method for assessing pulmonary function in horses. The relative variation and the reliability of observations within measurement sessions were calculated for expiratory tidal volume, for expired CO2 volume (%) and for the phase II and phase III slopes of the SBD-CO2 in horses without pulmonary disease and in horses suffering from differing degrees of chronic obstructive pulmonary disease. The coefficient of variation of expiratory tidal volume ranged from 0.23 to 0.32 and that of the expiratory CO2 volume from 0.19 to 0.43. The reliability of the measurements can be considered as good for expiratory tidal volume, and as excellent for expiratory CO2. Clinical status had little influence on the descriptive parameters or on the sample statistics of the expiratory tidal volume and expiratory CO2. The slopes of the SBD-CO2 curves had high variation and only moderate to good reliability. Clinical status had a considerable influence on the variation in the slopes of phase II.