Past research has characterized the size and circumstances of the uninsured population in fairly static terms. In particular, survey data have provided both annual and point-in-time estimates of the number of uninsured persons, but little is known about the transition between insured and uninsured states. Data limitations have also precluded a full examination of the pattern of health insurance loss over an extended period of time, in terms of the duration of loss of coverage, the degree to which persons reacquire health insurance, and the frequency of multiple spells of health insurance loss. These dynamic aspects of the changes in health insurance status are examined for a cohort of privately insured persons over a 32 month period using the 1984 panel of the Survey of Income and Program Participation (SIPP). The experience of this group is compared to a cohort of persons uninsured at the beginning of the SIPP panel. Our analysis reveals that the uninsured population is quite heterogeneous, consisting of many persons who lose coverage for relatively short periods of time, others who experience periodic spells without insurance coverage, and many who are persistently uninsured.