BackgroundThis study aimed to estimate the prevalence of pre-existing lung diseases in patients with lung cancer compared to people without lung cancer and examine the association between income levels and pre-existing lung diseases.MethodsData on patients with lung cancer (case) and the general population without lung cancer (non-cancer controls) matched by age, sex and region were obtained from the Korea National Health Insurance Service—National Health Information Database (n=51 586). Insurance premiums were divided into quintiles and medicaid patients. Conditional logistic regression models were used to examine the association between pre-existing lung diseases and the risk of lung cancer. The relationship between income level and the prevalence of pre-existing lung disease among patients with lung cancer was analysed using logistic regression models.ResultsThe prevalence of asthma (17.3%), chronic obstructive lung disease (COPD) (9.3%), pneumonia (9.1%) and pulmonary tuberculosis (1.6%) in patients with lung cancer were approximately 1.6–3.2 times higher compared with the general population without lung cancer. A significantly higher risk for lung cancer was observed in individuals with pre-existing lung diseases (asthma: OR=1.36, 95% CI 1.29 to 1.44; COPD: 2.11, 95% CI 1.94 to 2.31; pneumonia: 1.49, 95% CI 1.38 to 1.61; pulmonary tuberculosis: 2.16, 95% CI 1.75 to 2.66). Patients with lung cancer enrolled in medicaid exhibited higher odds of having pre-existing lung diseases compared with those in the top 20% income level (asthma: OR=1.75, 95% CI 1.56 to 1.96; COPD: 1.91, 95% CI 1.65 to 2.21; pneumonia: 1.73, 95% CI 1.50 to 2.01; pulmonary tuberculosis: 2.45, 95% CI 1.78 to 3.36).ConclusionsPre-existing lung diseases were substantially higher in patients with lung cancer than in the general population. The high prevalence odds of pre-existing lung diseases in medicaid patients suggests the health disparity arising from the lowest income group, underscoring a need for specialised lung cancer surveillance.