A study was undertaken to compare the size of tracheal tube used for paediatric anaesthesia derived from either the diameter of the distal phalanx of the little or index finger, or from a standard formula (age in years/4 + 4.5 mm). The results showed that the diameter of the terminal phalanx was a poor predictor of the external diameter of the tube that provided the best fit. The mean (SD) diameters were 9.34 (1.02), 10.24 (1.23) and 7.56 (0.97) mm for the little finger, index finger and tracheal tube providing ‘best fit’, respectively. The formula provided a better correlation with the internal diameter of the chosen tracheal tube. The mean (SD) figures were 5.61 (0.75) and 5.70 (0.67) mm for the formula and the chosen tracheal tube, respectively. The use of the diameter of the terminal phalanx of either the little or the index finger is an unreliable measurement for the prediction of tracheal tube size in paediatrics.