BACKGROUND: There is a need to reduce delays to diagnosis for chronic breathlessness to improve patient outcomes. OBJECTIVE: To conduct a mixed-methods feasibility study of a larger cluster randomised controlled trial (cRCT) investigating a structured symptom-based diagnostic approach versus usual care for chronic breathlessness in primary care. METHODS: 10 general practitioner practices were cluster randomised to a structured diagnostic approach for chronic breathlessness including early parallel investigations (intervention) or usual care. Adults over 40 years old at participating practices were eligible if presenting with chronic breathlessness without an existing explanatory diagnosis. The primary feasibility outcomes were participant recruitment and retention rate at 1 year. Secondary outcomes included number of investigations at 3 months, and investigations, diagnoses and patient-reported outcome measures (PROMs) at 1 year. Semistructured interviews were completed with patients and clinicians, and analysed using thematic analysis. RESULTS: Recruitment rate was 32% (48/150): 65% female, mean (SD) age 66 (11) years, body mass index 31.2 kg/m CONCLUSIONS: A cRCT investigating a symptom-based diagnostic approach for chronic breathlessness is feasible in primary care showing potential for timely investigations and diagnoses, with PROMs potentially indicating patient-level benefit. A further refined fully powered cRCT with health economic analysis is needed.