The goal of the National Medical Care Utilization and Expenditure Survey (NMCUES) of 1980 was to improve the understanding of the ways in which Americans use and pay for health care. This report is one in a series of descriptive reports based on NMCUES data. Data concerning insurance coverage were collected from household respondents in NMCUES. These data included the kind of insurance in effect for each person, the services covered, and the amounts paid by each source. In addition, the administration of private insurance plans and the kinds of charges covered were identified. The purpose of this report is to provide descriptive information about supplemental insurance coverage among the aged Medicare population with special emphasis on private supplemental health insurance coverage. For this report, supplemental insurance is defined as coverage (i.e., Medicaid, private, or other) in addition to Medicare and is to be distinguished from the Supplementary Medical Insurance part of Medicare that is known as SMI or Part B of Medicare. The results presented are based on data collected about the civilian, noninstitutionalized persons in the NMCUES national household sample who at any time during the survey year of 1980: (1) were 65 years of age or over, and (2) reported having been covered by Medicare Hospital Insurance (HI), or Medicare Supplementary Medical Insurance (SMI), or both. This report uses time-adjusted estimates that assign a single individual to different categories of insurance coverage according to the proportion of the year that he or she was covered by each kind of insurance. Consequently, estimates are made for person-years of coverage although they are expressed as persons for convenience. Approximately 4 out of 5 aged Medicare beneficiaries reported having some kind of insurance coverage in addition to Medicare during 1980. Approximately 67 percent of the aged Medicare population are estimated to have had private insurance in addition to Medicare; an estimated 13 percent had Medicaid. (Both of these estimates include 2.5 percent who reported Medicaid and private insurance simultaneously.) About 21 percent of the aged Medicare beneficiaries reported that Medicare was their only source of third-party coverage. The percentage of the aged Medicare beneficiaries who reported Medicare as their only source of third-party coverage was consistently 20 percent regardless of health status. However, the distribution among insurance categories of the remaining 80 percent who reported supplemental coverage of some type varied by health status. Medicare beneficiaries who were in poor health were much more likely to have Medicaid than Medicare beneficiaries who reported being in excellent health.(ABSTRACT TRUNCATED AT 400 WORDS)