National estimates of antiretroviral therapy (ART) adherence and adherence support services utilization are needed to inform efforts to improve the health of HIV-infected persons in the United States. In a nationally representative sample of HIV-infected adults receiving medical care, 86% self-reported taking all ART doses in the past 72 hours. Overall, 20% reported using adherence support services and 2% reported an unmet need for services. If all nonadherent persons not receiving adherence support and all persons with a self-perceived unmet need for adherence support accessed services, resources to support ∼42,673 additional persons would be needed. Factors associated with lower adherence included younger age, female gender, depression, stimulant use, binge alcohol use, greater than once-daily dosing, longer time since HIV diagnosis, and patient beliefs. Predictors of adherence are multifactorial so multiple targeted strategies to improve adherence are warranted. Providing adherence support services to all those in need may require additional resources.