This article discusses the utilization patterns of the Medicare aged who were continuously enrolled in the program for over a 4-year period. Data used for this study were compiled from a random sample of Medicare beneficiaries in Colorado who were continuously enrolled from October 1974 through December 1978. Five utilization variables: medical office visits, medical relative value units (RVUs), surgical RVUs, inpatient days, and covered dollars were examined in the article. Results from this study are consistent with previous longitudinal analyses. Consistent high users of services who consume substantial proportions of total expenditures and consistent nonusers having no or few services were found. During an average year of the 4-year period, it was observed that 18 per cent of the beneficiaries accounted for 88 per cent of the cost of services delivered. Because these high users tend to remain high users over time--more than one third of the high users in a given year continue to be high users during the following year--the findings here suggest that cost-containment strategies targeted to these high users may have a strong impact on overall Medicare program costs. Multivariate analysis is also conducted in this article so that the determinants of utilization patterns over time could be examined. Of particular interest is the finding that seasonal and trend variables play an important role in determining utilization over time.