To investigate the impact of the inhalation effort on the peak inspiratory flow through Turbuhaler, 100 asthmatics having a wide range of asthma severity (baseline forced expiratory volume in one second 28-127% of predicted normal) were studied. Each patient inhaled through four modifications of empty Turbuhaler inhalers, using first a "deep" inhalation and then a "forceful and deep" inhalation manoeuvre. Peak inspiratory flow increased by an average of 20% using a "forceful and deep" as compared to a "deep" inhalation, with a markedly higher increase for the patients who had a low peak inspiratory flow using the deep inhalation. Virtually all patients (97-100%) attained a peak inspiratory flow > or = 40 L x min(-1) after a "forceful and deep" inhalation. This study demonstrates that instructing the patient to take a "forceful and deep" inhalation optimizes the use of Turbuhaler. Irrespective of asthma severity, the vast majority of patients could attain a sufficiently high peak inspiratory flow with a "forceful and deep" inhalation.