Objectives. To determine whether the 2014 Medicaid expansions facilitated by the Affordable Care Act affected overall and early-stage cancer diagnosis for nonelderly adults.

Methods. We used Surveillance, Epidemiology, and End Results Cancer Registry data from 2010 through 2014 to estimate a difference-in-differences model of cancer diagnosis rates, both overall and by stage, comparing changes in county-level diagnosis rates in US states that expanded Medicaid in 2014 with those that did not expand Medicaid.

Results. Among the 611 counties in this study, Medicaid expansion was associated with an increase in overall cancer diagnoses of 13.8 per 100 000 population (95% confidence interval [CI] = 0.7, 26.9), or 3.4%. Medicaid expansion was also associated with an increase in early-stage diagnoses of 15.4 per 100 000 population (95% CI = 5.4, 25.3), or 6.4%. There was no detectable impact on late-stage diagnoses.

Conclusions. In their first year, the 2014 Medicaid expansions were associated with an increase in cancer diagnosis, particularly at the early stage, in the working-age population.

Public Health Implications. Expanding public health insurance may be an avenue for improving cancer detection, which is associated with improved patient outcomes, including reduced mortality.