BACKGROUND: Recent guidelines advocate early antiretroviral therapy (ART) to decrease human immunodeficiency virus (HIV) morbidity and prevent transmission, but suboptimal engagement in care may compromise impact. We sought to determine the economic and epidemiologic impact of incomplete engagement in HIV care in the United States. METHODS: We constructed a dynamic transmission model of HIV among US adults (aged 15-65 years) and conducted a cost-effectiveness analysis of improvements along the HIV care continuum : We evaluated enhanced HIV testing (annual for high-risk groups), increased 3-month linkage to care (to 90%), and improved retention (50% relative reduction in yearly disengagement and 50% increase in reengagement). Our primary outcomes were HIV incidence, mortality, costs and quality-adjusted life-years (QALYs). RESULTS: Despite early ART initiation, a projected 1.39 million (95% uncertainty range [UR], 0.91-2.2 million) new HIV infections will occur at a (discounted) cost of $256 billion ($199-298 billion) over 2 decades at existing levels of HIV care engagement. Enhanced testing with increased linkage has modest epidemiologic benefits and could reduce incident HIV infections by 21% (95% UR, 13%-26%) at a cost of $65 700 per QALY gained ($44 500-111 000). By contrast, comprehensive improvements that couples enhanced testing and linkage with improved retention would reduce HIV incidence by 54% (95% UR, 37%-68%) and mortality rate by 64% (46%-78%), at a cost-effectiveness ratio of $45 300 per QALY gained ($27 800-72 300). CONCLUSIONS: Failure to improve engagement in HIV care in the United States leads to excess infections, treatment costs, and deaths. Interventions that improve not just HIV screening but also retention in care are needed to optimize epidemiologic impact and cost-effectiveness.