Background Bronchoscopic limitations in reaching peripheral pulmonary lesions (PPLs) can compromise biopsy sensitivity, especially for small PPLs. Therefore, we developed the balloon dilatation for bronchoscope delivery (BDBD) technique to dilate bronchial pathways and facilitate bronchoscope advancement into the periphery. This study evaluated the diagnostic performance and safety profile of transbronchial biopsy using this technique. Methods This multicentre, single-arm, prospective study included patients with bronchus sign-positive PPLs measuring <20 mm. Bronchoscopy was performed using ultrathin or thin bronchoscopes under conscious sedation. When the bronchoscope could not advance further, the BDBD technique was used to approach closer to the target, followed by biopsies. The primary endpoint was the diagnostic sensitivity for malignancy in specimens obtained through the specified procedure, defined as bronchoscope advancement using balloon dilatation, direct biopsy site visualisation and absence of serious adverse events. Results Eighteen of 22 patients who underwent bronchoscopy with the BDBD technique were ultimately diagnosed with cancer. BDBD enabled bronchoscope advancement in all 18 cases without serious complications, allowed direct biopsy site visualisation in 17 and detected cancer in 14. Thus, the diagnostic sensitivity for malignancy was 77.8% (14/18). Beyond these cases, one patient who met all procedural criteria was diagnosed with cryptococcosis. Another patient was diagnosed with cancer without direct visualisation. On average, BDBD enabled bronchoscope advancement by 2.3 bifurcations. Conclusion In this small observational study, BDBD appeared to be a promising technique for improving the diagnostic sensitivity of bronchoscopy for small PPLs. Further validation is necessary in large cohorts. Trial registration number jRCT2052220174