The prevalence of disability at the age of 75+ measured by the Katz Index of Activities of Daily Living (ADL) was compared among three birth cohorts: those born < or = 1903 (n = 348), those born < or = 1913 (n = 586), and those born < or = 1923 (n = 758). Significant risk factors for disability were female sex and age; the cohort effect was not significant. The prevalence rates of disability were 29.0% (95% CI 24.2-33.8), 34.8% (30.9-38.7), and 28.8% (25.5-32.0) for the first, second, and third cohorts. In the age group 75-79 years the rates were 20.1% (95% CI 13.8-26.4), 25.5% (20.2-30.7), and 14.4% (10.6-18.1). The change was due to the declining disability of women. The distributions in the three cohorts based on the numbers of ADL limitations did not differ. As far as the whole aged populations were concerned, longer life was not accompanied by improving health.