In order to evaluate whether children can be taught the efficient use of a pressurized terbutaline aerosol with a tube spacer 71 children were given careful instruction in aerosol inhalation technique according to the manufacturer's instructions. Inhalation technique was assessed as being efficient when a child achieved an increase of more than 19% in FEV1 10 min after taking two puffs of terbutaline (each puff = 0.25 mg). After instruction about 37% of children aged 5-7 years and about 80% of children over 7 years were efficient in inhalation technique. Inhalation through the nose after actuation into the mouth accounted for about 50% of treatment failures, with the problem being more frequent in the younger age group. When this error was corrected about 83% of the children were efficient in the technique. Coordination problems and too rapid inspiration after actuation were also common errors. The findings stress the importance of checking all children's inhalation technique before prescribing inhalation therapy. Careful instruction was not enough. The possibility of nasal inhaling should be borne in mind when looking for causes of treatment failure.