OBJECTIVE: To examine the importance of continuous health insurance for access to care by comparing the access and cost experiences of insured adults with a recent time uninsured to the experiences of currently uninsured adults and experiences of adults with no time uninsured within a reference time period (continuously insured). DATA SOURCES: Adults ages 18-64. Data draw from three different survey databases: the Robert Wood Johnson Foundation 1996-1997 Community Tracking Survey, the Kaiser/Commonwealth 1997 National Survey of Health Insurance, and the 1995-1997 Kaiser/Commonwealth State Low Income Surveys. STUDY DESIGN: The study groups individuals into three insurance categories based on respondents' reports of insurance coverage within a reference time period: continuously insured; insured when surveyed but with recent time uninsured; and currently uninsured. In the two Kaiser/Commonwealth surveys the recently uninsured group included any insured respondent with a time uninsured in the past two years. In the Community Tracking Survey, the recently uninsured group included any insured respondent with a time uninsured in the past year. Measures of access include foregoing health care when needed, usual source of care, use of health care services, difficulties paying for medical care, and satisfaction with care. DATA COLLECTION: All three surveys were conducted primarily by telephone. The Community Tracking Survey drew from 60 community sites, with an additional random national sample. The Kaiser/Commonwealth National Survey was a random national sample; the Kaiser/Commonwealth State Low Income Surveys included adults ages 18-64 with incomes at or below 250 percent of poverty in seven states: Minnesota, Oregon, Tennessee, Florida, Texas, New York, and California. PRINCIPAL FINDINGS: Compared to the continuously insured, those insured but with a recent time uninsured were at high risk of going without needed care and of having problems paying medical bills. This group was two to three times as likely as those with continuous coverage to report access problems. Rates of access and cost problems reported by insured adults with a recent time uninsured neared levels reported by those who were uninsured at the time of the survey. These two groups also rated care received more negatively than did adults with continuous insurance coverage. In general, the access gap between persons insured and uninsured widened as a result of distinguishing insured adults with a recent time uninsured from insured adults with no time uninsured. CONCLUSION: Studies that focus on current insurance status alone will underestimate the extent to which having a time uninsured during the year contributes to access difficulties and undermines quality of care, and will underestimate the proportion of the population at risk because they are uninsured. Policy reforms are needed to maintain continuous insurance coverage and avoid spells uninsured. Currently uninsured and unstably insured adults are both at high risk.