We report the case of a defective Tec 6 vaporizer (Datex-Ohmeda) which delivered higher concentrations of desflurane (end-tidal concentration: 3.5 vol%) than indicated by the vaporizer dial setting (2 vol%), at a fresh gas flow of 1 L.min-1. This condition was caused by a defective rotary control valve. Therefore it is essential to administer desflurane only in presence of a vapour analyser.