256 asthmatic children receiving regular inhalation therapy demonstrated how they used their inhalers. Pulmonary function measurements (PFM) were made before and after the demonstrations, and errors in technique were recorded. 242 children had reversible airway obstruction on the day of study. In only 109 (45%) did the inhalation result in an increase in FEV1 greater than or equal to 15% (efficient technique). An efficient inhalation technique was found in 46% of children who demonstrated a pressurized aerosol, 59% who demonstrated a tube spacer aerosol and 46% who demonstrated a rotahaler, and the frequency of efficient technique varied from 17% to 84% between six different groups of instructors. 87% of children controlled and 25% not controlled with PFM at the time of prescription had an efficient technique. Children under 6 years had a more inefficient and a more faulty technique than older children, but otherwise age did not influence the result. Neither was time since instruction of any importance for efficiency or number of errors. The errors recorded that seem to influence efficiency most were: coordination problems, rapid inspirations, ceasing to inspire when the aerosol was fired, and inhalation through the nose. The results emphasize the paramount importance of clear instructions and control of inhalation technique at the time the treatment is prescribed.