With a handheld peak inspiratory flow (PIF) meter (In-Check DIAL), the internal resistance of the Diskus (DKR) and the Turbuhaler (THR) can be simulated by means of calibrated resistances. This study investigated whether patients with asthma or chronic obstructive pulmonary disease could generate a PIF which is optimal for the Diskus (DK) (30 l/min) and theTurbuhaler (TH) (60 l/min). Peak expiratory flow (PEF) and maximal inspiratory pressure (MIP) were also assessed. All 50 patients (30 males, mean age 56.9) achieved the required flow of 30 l/min with the DKR, while six patients (five females) could not generate the optimal flow of 60 l/min via theTHR. Analysis showed that MIP was an independent predictor of an adequate PIF through theTHR. During the study eight patients were treated for an exacerbation of which four could not generate an adequate PIF via the THR. Analysis showed that the female gender and an exacerbation appeared to be independent predictors of the inability to generate an adequate PIF via the THR. Twelve per cent of patients (25% of females) could not generate the optimal inspiratory flow via the THR. When in doubt whether the patient can generate an adequate inspiration, measurement with the In-Check DIAL is recommended.