The law now covers preventive screenings, and the federal government helps pay for them by funding Medicaid expansion in states that opted in. It is unclear how the expansion of Medicaid affects the detection stage of colon cancer. This study aimed to examine whether there was a difference over time in the percentage of patients diagnosed with early-stage colon cancer between states that expanded Medicaid and states that did not. It is a cohort study that takes a snapshot of time. The National Cancer Database was used to analyze multicenter registry data (2006–2016). Newly diagnosed colon cancer patients in Medicaid expansion and non-expansion states in 2014 numbered 25,462. Researchers in this study compared the annual percentage of patients with stage I-II colon cancer to those with stage III-IV disease. There were a total of 10,289 patients were found in the states that participated in the expansion, and 15,173 were found in the states that did not. Individuals in each cohort were roughly the same age (median = 55) and gender (46.7% female). Patients in non-expansion states were more likely to be black (33.4% versus 24.0%), to live in a zip code with a median income of $38,000 (39.7% versus 28.2%), and to have lower levels of education (37.4% versus 28.2%). Expansion and non-expansion cohorts had similar percentages of patients with early-stage colon cancer in 2006. After 2014, the percentage of patients diagnosed with colon cancer at an early stage fell by 0.8% annually in non-expansion states and rose by 0.9% annually in expansion states (P<0.05). Using a propensity score, the absolute difference between the 2 groups in 2016 was 8.8% points (39.7 vs. 30.9, P<0.001). Information for the National Cancer Database comes exclusively from facilities recognized by the Commission on Cancer; it is not representative of the general population. Within expansion states, the percentage of patients diagnosed and treated for early-stage colon cancer at Commission on Cancer-accredited facilities increased after Medicaid expansion in 2014, while the percentage increased in non-expansion states. Patients in expansion states may have benefited from quicker diagnoses because of increased insurance coverage.