BackgroundIt remains unclear whether the racial-ethnic composition or the socioeconomic profiles of eligible primary care practices better explain practice participation in the Centers for Medicare and Medicaid Services' (CMS) Comprehensive Primary Care Plus (CPC+) program.ObjectiveTo examine whether practices serving high proportions of Black or Latino Medicare fee-for-service (FFS) beneficiaries were less likely to participate in CPC+ in 2021 compared to practices serving lower proportions of these populations.Design2019 IQVIA OneKey data on practice characteristics was linked with 2018 CMS claims data and 2021 CMS CPC+ participation data. Medicare FFS beneficiaries were attributed to practices using CMS's primary care attribution method.Participants11,718 primary care practices and 7,264,812 attributed Medicare FFS beneficiaries across 18 eligible regions.MethodsMultivariable logistic regression models examined whether eligible practices with relatively high shares of Black or Latino Medicare FFS beneficiaries were less likely to participate in CPC+ in 2021, controlling for the clinical and socioeconomic profiles of practices.Main measuresProportion of Medicare FFS beneficiaries attributed to each practice that are (1) Latino and (2) Black.Key resultsOf the eligible practices, 26.9% were CPC+ participants. In adjusted analyses, practices with relatively high shares of Black (adjusted odds ratio, aOR = 0.62, p < 0.05) and Latino (aOR = 0.32, p < 0.01) beneficiaries were less likely to participate in CPC+ compared to practices with lower shares of these beneficiary groups. State differences in CPC+ participation rates partially explained participation disparities for practices with relatively high shares of Black beneficiaries, but did not explain participation disparities for practices with relatively high shares of Latino beneficiaries.ConclusionsThe racial-ethnic composition of eligible primary care practices is more strongly associated with CPC+ participation than census tract-level poverty. Practice eligibility requirements for CMS-sponsored initiatives should be reconsidered so that Black and Latino beneficiaries are not left out of the benefits of practice transformation.