Decades-old data indicate that people imprisoned in the US have poor access to health care despite their constitutional right to care. Most prisons impose co-payments for at least some medical visits. No recent national studies have assessed access to care or whether co-pays are associated with worse access. To determine the proportion of people who are incarcerated with health problems or pregnancy who used health services, changes in the prevalence of those conditions since 2004, and the association between their state's standard prison co-payment and care receipt in 2016. This cross-sectional analysis was conducted in October 2023 and used data from the Bureau of Justice Statistics' 2016 Survey of Prison Inmates, a nationally representative sample of adults in state or federal prisons, with some comparisons to the 2004 version of that survey. The state's standard, per-visit co-payment amount in 2016 compared with weekly earnings at the prison's minimum wage. Self-reported prevalence of 13 chronic physical conditions, 6 mental health conditions, and current severe psychological distress assessed using the Kessler Psychological Distress Scale; proportion of respondents with such problems who did not receive any clinician visit or treatment; and adjusted odds ratios (aORs) comparing the likelihood of no clinician visit according to co-payment level. Of 1 421 700 (unweighted: n = 24 848; mean [SD] age, 35.3 [0.3] years; 93.2% male individuals) prison residents in 2016, 61.7% (up from 55.9% in 2004) reported 1 or more chronic physical conditions; among them, 13.8% had received no medical visit since incarceration. A total of 40.1% of respondents reported ever having a mental health condition (up from 24.5% in 2004), of whom 33.0% had received no mental health treatment. A total of 13.3% of respondents met criteria for severe psychological distress, of whom 41.7% had not received mental health treatment in prison. Of state prison residents, 90.4% were in facilities requiring co-payments, including 63.3% in facilities with co-payments exceeding 1 week's prison wage. Co-payments, particularly when high, were associated with not receiving a needed health care visit (co-pay ≤1 week's wage: aOR, 1.43; 95% CI, 1.10-1.86; co-pay >1 week's wage: aOR, 2.17; 95% CI, 1.61-2.93). This cross-sectional study found that many people who are incarcerated with health problems received no care, particularly in facilities charging co-payments for medical visits.