Epidemiology of disability in the oldest old: methodologic issues and preliminary findings.

Little is known about the health problems of the fastest-growing segment of the population. Three major community-based studies present prevalence information on physical and mental disabilities among the oldest old. These data also illustrate the limitations of interpretations in cross-sectional studies, and emphasize the value of prospective investigations of populations aged 85 and older. Although more costly and time-consuming, such prospective longitudinal studies are indispensable to sound policy planning.