Background: COPD is one of the most common comorbidities among patients with lung cancer, and an important determinant of their survival. While COPD is frequently under diagnosed in the general population, less is known about the occurrence of COPD underdiagnosis in patients with lung cancer. Objectives: 1) To estimate prevalence and under diagnosis of COPD among patients newly diagnosed with lung cancer. 2) To quantify the association between timing of COPD diagnosis and stage of lung cancer at diagnosis and whether this association is modified by sex, race, and socioeconomic status (SES). Methods: A secondary data analysis was conducted using the SEER-Medicare database. We included Medicare beneficiaries aged 66+years with primary invasive lung cancer diagnosed from 2008 to 2017 who were alive at the time of cancer diagnosis and had continuous Medicare coverage 12 months prior and 3 months after lung cancer diagnosis. Exposure: Comorbid COPD was determined using Medicare claims and classified as early COPD (evidence of COPD related healthcare utilization (HCU) >3 months before lung cancer diagnosis) versus late COPD (evidence of COPD related HCU only during the +/- 3 months from lung cancer diagnosis). Outcome: Early (localized) versus late (regional and distant) stage of lung cancer diagnosis. Statistical analysis: We used generalized linear models to estimate adjusted absolute (prevalence difference PD) and relative (prevalence ratio PR) association. Results: In a population of 185,405 Medicare beneficiaries with lung cancer, 131,230 (71%) had evidence of COPD. Compared to lung cancer patients without COPD, those with COPD were more likely to be younger, male, White, of lower SES, have more comorbidities, higher HCU, and be diagnosed with early stage lung cancer. Patients with early COPD diagnosis constitute 66,867 (51%) of patients with COPD and lung cancer. They were more likely to be older, female, Non-Hispanic White, of lower SES, and to have more comorbidities, acute exacerbation of COPD, and more early-stage lung cancer compared to patients with late COPD diagnosis. We observed a significant positive association between early COPD diagnosis and early-stage lung cancer diagnosis (PR= 1.27, 95% CI= 1.24 to 1.30; PD= 0.062, 95% CI= 0.057 to 0.066) in models adjusted for age, sex, race, SES, HCU, comorbidities, year of cancer diagnosis, SEER region, and histology. In subgroup analyses, this association was stronger for males, Non-Hispanic Black, American Indian Alaskan Native, and patients of lower SES. Conclusions: Approximately seven out of 10 patients with lung cancer have COPD, and late COPD diagnosis is common. Earlier COPD diagnosis is associated with diagnosis of lung cancer at earlier stages. These findings highlight the need for more structured screening of COPD among individuals at high risk for lung cancer.