Lifetime-certified diagnostic radiologists whose Maintenance of Certification (MOC) was not mandated by the American Board of Radiology (ABR) were far less likely to participate in ABR MOC programs—especially general radiologists and those working in smaller, nonacademic practices in states with lower population densities. Defining diagnostic radiologists as those whose only ABR certificate is in diagnostic radiology, lead author Andrew B. Rosenkrantz of NYU Langone Medical Center and colleagues cross-referenced Medicare and Medicaid data with ABR’s own public search engine to determine that 11,479 of 20,354 total diagnostic radiologists (56.4%) participated in MOC. Although diagnostic radiologists with time-limited certificates nearly universally participate in ABR MOC—99.6% (10,058/10,099)—participation rates were only 13.9% (1421/10,225) among the cohort with lifetime ABR certificates. “Many opinions have been expressed regarding MOC in radiology,” says Rosenkrantz, the 2017 ARRS Leonard Berlin Scholar. “But there is actually very little public data on the matter. Through this work, we hope to bring objective findings to help inform the discussions.” The rates of nonmandated participation were higher (all p < 0.001) for academic than for nonacademic radiologists (28.0% vs 11.3%), subspecialists than for generalists (17.0% vs 11.5%), and those in larger practice groups (< 10 members, 5.0%; 10–49 members, 12.6%; ≥ 50 members, 20.7%). State-level rates of nonmandated participation varied from 0.0% (South Dakota, Montana) to 32.6% (Virginia) and positively correlated with state population density (r = 0.315).