BACKGROUND: Lung cancer remains the leading cause of cancer-related deaths globally, with gradual improvements in patient survival attributed to early detection through low-dose computed tomography screening and advances in oncological therapies. Despite these advancements, the management of comorbidities, particularly cardiovascular disease and chronic obstructive pulmonary disease, is critical due to their shared causal link with lung cancer - smoking. This study explores the prevalence of comorbidities among lung cancer patients in Denmark over four decades, using comprehensive national registry data. METHODS: By examining the Danish National Patient Register and Danish Cancer Registry, we identified all Danish lung cancer cases diagnosed from 1980 to 2018, analyzing comorbidities and causes of death. A comparison cohort matched by age, sex, municipality, and marital status was also established. FINDINGS: The findings reveal a significant increase in comorbidities among lung cancer patients over time, while this increase was less significant in the comparison cohort. Almost half of lung cancer patients had at least one comorbidity in the most recent period, 2008-2018. Cardiovascular disease, chronic obstructive pulmonary disease, diabetes, stroke, and peripheral atherosclerosis were the most prevalent comorbidities. Among patients diagnosed with lung cancer, it was the cause of death in 84 % of cases. The study also highlights a notable decrease in deaths from ischemic heart disease, with an increase in dementia-related deaths, suggesting an increasing burden of neurodegenerative diseases in aging populations. INTERPRETATION: This longitudinal analysis highlights that as the burden of comorbidities increases, comprehensive management strategies become increasingly crucial. These strategies could include less invasive diagnostic approaches, such as endobronchial evaluation, as well as treatment options like segmental resection and stereotactic body radiation. Addressing comorbidities alongside cancer treatment may improve patient outcomes and overall quality of life in aging populations.