Height is the only factor used to differentiate horses and ponies for competitive purposes; any horse without shoes that is less than 148 cm at the highest point of the withers can be classified as a pony. However, variations in height associated with anatomical and other factors can occur. In this study the accuracy of the most commonly used measuring procedure and seven independent factors that might affect apparent height were evaluated in skeletally mature horses that were less than 155 cm in height. The results indicated that there was an improvement of 57 per cent in the accuracy of measurement when a second spirit level was added to the traditional measuring stick. The spread of values recorded for a series of repeated measurements by one person was ±1·06 cm, but the spread increased to ±1·89 cm when three different people made the measurements. Some variation in height occurred when the position of the pony's head was changed, but none occurred after a period of exercise. There was a reduction in height of 0·84 cm when the animals were allowed to adapt to their surroundings, and there was some reduction after they had been tranquillised.