Analysis of 1998-2000 health insurance data from the Medical Expenditure Panel Survey shows large numbers of people with unstable health insurance coverage. Young adults, Hispanics, people with low levels of education, those who transition into and out of poverty, and those with private non-group insurance are most likely to have unstable coverage. In addition, demographic factors and type of insurance interact to determine stability of coverage. Young adults and Hispanics with Medicaid or private insurance, for example, were relatively likely to lose their coverage. And less than half of people who transitioned into and out of low income and were initially uninsured were able to obtain coverage. Policies must target these high-risk groups in order to provide them with stable health insurance coverage.