Background: No recent national studies have assessed chronic illness prevalence or access to care among persons without insurance in the United States. Objective: To compare reports of chronic conditions and access to care among U.S. adults, by self-reported insurance status. Design: Population-based survey. Setting: National Health and Nutritional Examination Survey (1999–2004). Participants: 12 486 patients age 18 to 64 years. Measurements: Estimates of national rates of cardiovascular disease, hypertension, diabetes, hypercholesterolemia, active asthma or chronic obstructive pulmonary disease, previous cancer, and measures of access to care. Results: On the basis of National Health and Nutrition Examination Survey (1999–2004) responses, an estimated 11.4 million (95% CI, 9.8 million to 13.0 million) working-age Americans with chronic conditions were uninsured, including 16.1% (CI, 12.6% to 19.6%) of the 7.8 million with cardiovascular disease, 15.5% (CI, 13.4% to 17.6%) of the 38.2 million with hypertension, and 16.6% (CI, 13.2% to 20.0%) of the 8.5 million with diabetes. After the authors controlled for age, sex, and race or ethnicity, chronically ill patients without insurance were less likely than those with coverage to visit a health professional (6.2% vs. 22.6%) and have a standard site for care (6.2% vs. 26.1%) and more likely to identify an emergency department as their standard site for care (7.1% vs. 1.1%) (P 0.001 for all comparisons). Limitation: The study was cross-sectional and used self-reported insurance and disease status. Conclusion: Millions of U.S. working-age adults with chronic conditions do not have insurance and have poorer access to medical care than their insured counterparts.