A radioaerosol technique has been developed in order to assess deposition patterns from a new metered dose powder inhaler (Turbuhaler, Astra Pharmaceuticals). The radionuclide Tc99m dissolved in chloroform was added to a spheronised formulation of micronised terbutaline sulphate and the chloroform was allowed to evaporate. Turbuhaler subsequently delivered 0.5 mg of treated drug per metered dose. In vitro tests with a multistage liquid impinger showed that the fractionation of the drug dose between different particle size bands was similar to the fractionation of radioactivity. In a group of ten asthmatic patients, a mean 14.2% (SEM 2.1) of the drug dose was deposited in the lungs, with 71.6% (3.0) of the dose in the oropharynx. Of the remainder, 13.7% (2.1) was deposited on the mouthpiece, and 0.5% (0.2) recovered from exhaled air. The radiolabel was present in both central and peripheral zones of the lungs. All patients bronchodilated; forced expiratory volume in one second (FEV1) increased from 1.40 (0.24) l to 1.77 (0.24) l (p less than 0.01) 20 min after inhalation. These results suggest that both the distribution of drug and the clinical effect of terbutaline sulphate delivered from Turbuhaler are similar to those from a pressurised metered dose inhaler (MDI).