The British guidelines on asthma management and other recent reviews have widely advocated the use of spacer devices for asthmatic patients.^1-3 Spacer devices allow aerosol to be inhaled through a valved mouthpiece during a single inspiratory breath or as is often recommended for children during tidal breathing.² During the expiratory phase of a tidal breathing manoeuvre the valve closes. Although this action prevents leakage of aerosol from the spacer and ingress of moisture into the holding chamber, there is an inevitable increase in the expiratory resistance. We have measured the resistance to expiratory airflow in two commonly used spacer devices: the Volumatic (GlaxoWellcome) and the Nebuhaler (Astra-Zeneca) and found it to be high. Expiratory resistance was measured by passing air through the mouthpiece using a sealed connection. Two flow rates were chosen and measured using a Gap Rotameter: 30 l/min designed to mimic airflow during quiet expiration and 100 l/min to mimic . . . [Full text of this article]