Background: Most lung cancers present symptomatically, but the pathway to diagnosis in primary care can be complex and is poorly understood. Significant event audit (SEA) is a quality improvement technique widely used in UK general practice. Aim: To gain insights into the diagnostic process for lung cancer, drawn from analysis of SEA documents. Design and setting: Qualitative analysis of SEAs from 92 general practices in the North of England Cancer Network. Method: Participating practices were provided with a standardised electronic template and asked to undertake a significant event audit related to the most recent diagnosis of lung cancer in the practice, even if that patient had since died. Reported accounts for 132 diagnoses were analysed using a modified framework approach. Results: Most SEAs demonstrated timely recognition and referral. Where this had taken longer, there were often reasonable explanations, including: chest X-rays reported as normal or with benign findings; patient-mediated factors, such as delay in re-presenting or declining earlier referral; and presentation complicated by comorbidity. Some opportunities for earlier referral were also found. Lessons drawn from these events included limitations of chest X-ray as a diagnostic tool, the need for vigilance in patients with existing morbidity, and the importance of 'safety-netting'. Conclusion: Qualitative synthesis of SEAs offered considerable value in understanding circumstances surrounding the diagnostic process for lung cancer in primary care. The most common presentation was with cough or other symptoms indicative of infection, and it is by understanding more about these patients in particular that most can be gained. © British Journal of General Practice.