INTRODUCTION: Misdiagnosis in acute dyspneic patients (ADP) has consequences on their outcome. Lung ultrasound (LUS) is an accurate tool to improve diagnostic performance. The main goal of this study was to assess the determinants of increased diagnostic accuracy using LUS. MATERIALS: Multicentre, prospective, randomized study including emergency physicians and critical care physicians treating ADP on a daily basis. Each participant received three difficult clinical cases of ADP: one with only clinical data (OCD), one with only LUS data (OLD), and one with both. Ultrasound video loops of A, B and C profiles were associated with the cases. Which physician received what data for which clinical case was randomized. Physicians assessed the diagnostic probability from 0 to 10 for each possible diagnosis. The number of uncertain diagnoses (NUD) was the number of diagnoses with a diagnostic probability between 3 and 7, inclusive. RESULTS: Seventy-six physicians responded to the study cases (228 clinical cases resolved). Among the respondents, 28 (37%) were female, 64 (84%) were EPs, and the mean age was 37±8 years. The mean NUDs, respectively, when physicians had OCD, OLD, and both were 2.9±1.8, 2.2±1.7, 2.2±1.8 (p = 0.02). Ultrasound data and ultrasound frequency of use were the only variables related to the NUD. Higher frequency of ultrasound use by physicians decreased the number of uncertain diagnoses in difficult clinical cases with ultrasound data (OLD or associated with clinical data). CONCLUSION: LUS decreases the NUD in ADP. The ultrasound frequency of use decreased the NUD in ADP clinical cases with LUS data.