The present study was designed: (a) to compare the additional inspiratory laryngeal mask airway (LMA) resistance measured in vitro during simulated ventilation and in vivo in five anaesthetised and mechanically ventilated patients; and (b) to evaluate the resistive pressure drop along the length of the LMA. After the differential pressure across the mask was measured, the pressure-flow relationship was characterised by Rohrer's equation and in vitro and in vivo resistance was calculated. Thereafter, the distal pressure measuring point was moved along the length of the LMA and differential pressure was measured at each point under in vitro and in vivo conditions. Values for resistance were approximately twice as great in vivo as those obtained in vitro, with most of the resistive pressure drop occurring across the vertical bars, especially when measured in vivo. We conclude that in vivo positioning of the LMA significantly increases resistance because of the configurational changes occurring when the LMA is in situ.