Thirty patients hospitalized with asthma who had been taking aerosol bronchodilators from canister nebulizers were evaluated for their aerosol inhalation technique. Fourteen patients (47 percent) used an incorrect technique. The most frequent mistake was to inhale first, then to actuate the canister and breathhold. These patients were taught the correct technique. Some learned it easily but others had difficulty. With a teaching aid, incorporating a horn which is actuated by inspiration, all 14 patients learned to inhale the aerosol correctly. When retested five of the 10 patients had reverted to the old incorrect technique and required another lesson. Physicians who prescribe aerosol medication from a canister nebulizer should evaluate each patient for the aerosol inhalation technique. Those who inhale incorrectly should be taught repeatedly until they learn the correct technique and retain it. The teaching aid, utilizing an audio signal, is helpful in this training.