We studied 86 patients to determine the sources of error in patient use of pressurized aerosol inhalers (PAIs), the risk factors for poor technique, and the effect of instruction of patients on PAI technique. Patients were assessed before and after instruction using a standard procedure. Critical initial assessment showed that 31% of patients had an adequate technique. Seventy-five patients were reassessed after instruction, and 72% of these were determined to be adequate users. The most common errors were failure to breath-hold after-inhalation, failure to exhale prior to inhalation, and poor coordination of aerosol administration with the start of inhalation. Elderly patients were consistently poor users. Technique improved with more PAIs being used, but duration of PAI use was not a significant factor.