IMPORTANCE:Increasing numbers of older, community-dwelling adults have functional impairments that prevent them from leaving their homes. It is uncertain how many people who live in the United States are homebound. OBJECTIVES:To develop measures of the frequency of leaving and ability to leave the home and to use these measures to estimate the size of the homebound population in the US population. DESIGN, SETTING, AND PARTICIPANTS:Cross-sectional data from the National Health and Aging Trends Study collected in 2011 in the contiguous United States. Participants were a nationally representative sample of 7603 noninstitutionalized Medicare beneficiaries 65 years and older. MAIN OUTCOMES AND MEASURES:We defined homebound persons as those who never (completely homebound) or rarely (mostly homebound) left the home in the last month. We defined semihomebound persons as those who only left the home with assistance or had difficulty or needed help leaving the home. We compared demographic, clinical, and health care utilization characteristics across different homebound status categories. RESULTS:In 2011, the prevalence of homebound individuals was 5.6% (95% CI, 5.1%-6.2%), including an estimated 395,422 people who were completely homebound and 1,578,984 people who were mostly homebound. Among semihomebound individuals, the prevalence of those who never left home without personal assistance was 3.3% (95% CI, 2.8%-3.8%), and the prevalence of those who required help or had difficulty was 11.7% (95% CI, 10.9%-12.6%). Completely homebound individuals were more likely to be older (83.2 vs 74.3 years, P < .001), female (67.9% vs 53.4%, P < .006), and of nonwhite race (34.1% vs 17.6%, P < .001) and have less education and income than nonhomebound individuals. They also had more chronic conditions (4.9 vs 2.5) and were more likely to have been hospitalized in the last 12 months (52.1% vs 16.2%) (P < .001 for both). Only 11.9% of completely homebound individuals reported receiving primary care services at home. CONCLUSIONS AND RELEVANCE:In total, 5.6% of the elderly, community-dwelling Medicare population (approximately 2 million people) were completely or mostly homebound in 2011. Our findings can inform improvements in clinical and social services for these individuals.