It is well known that correct use of inhalers plays a critical role in optimal inhalation therapy, but the impact of incorrect inhaler use on pulmonary drug delivery has not been quantitatively evaluated. The aim of this study was to investigate the frequency of holding inhalers at incorrect angles during the drug-loading step while using Turbuhaler® and to quantify the influence of the inhaler angle on in vitro pulmonary delivery. Thirty patients prescribed Turbuhaler® at Shiga University of Medical Science Hospital were enrolled. During inhalation, the participants' inhalation techniques were assessed by clinical pharmacists. Additionally, the influence of the inhaler angle on pulmonary delivery of budesonide via Symbicort® Turbuhaler® was investigated using a Twin-Stage Liquid Impinger. Output efficiency (OE), stage 2 deposition (St2), and OE × St2 were calculated. An incorrect angle during the drug-loading step was observed in 33.3% of the participants. In vitro testing demonstrated that OE, an index of the loaded dose, significantly decreased by 73.3% at an incorrect angle, while St2, an index of the deagglomerating efficiency, was stable independent of the holding angle. OE × St2, indicating the bronchial and pulmonary drug delivery amount, decreased by 76.9%. An incorrect holding angle reduced the loaded dose, resulting in decreased pulmonary delivery. Error in the inhaler angle occurs frequently and demonstrates a considerable impact on pulmonary drug delivery. Hence, it is necessary to assess the Turbuhaler® angle during inhalation.