PURPOSE: This study examines the impact of Medicaid expansion under the Affordable Care Act on the stage of lung cancer diagnosis among rural and urban patients in New Hampshire. METHODS: Data from the New Hampshire State Cancer Registry spanning 2010-2019 were analyzed to compare lung cancer diagnosis stages before and after the July 2014 Medicaid expansion. Rural-urban categorization utilized Rural-Urban Continuum Codes, and logistic regression with difference-in-difference analysis assessed the differential effects of Medicaid expansion on late-stage diagnoses between rural and urban patients. FINDINGS: Post-expansion, there was a significant decrease in the proportion of late-stage lung cancer diagnoses statewide. Rural patients initially had higher rates of late-stage diagnoses compared to urban patients, but post-expansion, this disparity diminished significantly. Logistic regression indicated reduced odds of late-stage diagnosis among rural patients after expansion (OR = 0.719, P = .035), demonstrating a greater benefit in rural areas. CONCLUSIONS: Medicaid expansion in New Hampshire was associated with a substantial reduction in late-stage lung cancer diagnoses, particularly benefiting rural patients who historically faced higher barriers to health care access. These findings underscore the potential of Medicaid expansion to mitigate rural-urban disparities in cancer care outcomes.