While estimates of the country's uninsured vary, ranging from 10 to 18 percent of the general population, virtually every study on use of medical services reports that lack of health insurance represents a major barrier to medical care. Based on the 1986 national Robert Wood Johnson Access Survey of 10,130 noninstitutionalized persons, the characteristics of working-age adults without health insurance, and the consequences, are examined. Among working-age adults, the uninsured are most likely to be poor or near-poor, Hispanic, young, unmarried and unemployed. Compared with the insured, they have significantly fewer ambulatory visits during a year, are less likely to have contact with a medical provider during a 12-month period, and are more likely to receive their care in a hospital outpatient clinic or emergency room. Differences in health status do not account for these findings. Especially among persons with chronic and serious illnesses, the uninsured are less likely than the insured to receive medical care. Further, the uninsured are significantly more likely to report needing but not receiving medical care, primarily for economic reasons, and although poorer, they have higher out-of-pocket medical expenses than others in the population.