After resolution of conditions necessitating tracheostomy in children, decannulation may be extremely difficult and associated with significant morbidity. Endoscopy and radiography can identify anatomical abnormalities preventing decannulation. Physiological abnormalities are, however, more difficult to assess. A system has been developed whereby functional upper airway resistance can be calculated in children with a tracheostomy. The instrument used was a modified Mercury Electronics rhinomanometer. A probe was placed through the tracheostome and another probe was situated in an occluding anaesthetic face mask. Upper airway resistance was derived from the differential pressure and flow changes recorded in anaesthetized children who were breathing spontaneously. Studies in 26 children have revealed low airway resistance in all those successfully decannulated. Further studies will hopefully allow identification in advance of children who will have decannulation difficulties.