Intrinsic capacity (IC) is central to healthy aging, but its prognostic relevance among the oldest old remains unclear.

To investigate the associations of IC with disability and mortality, and to determine whether an exploratory cardio-renal measure adds prognostic information.

Prospective cohort study, with the two cohorts analyzed separately.

Community-dwelling older adults in Japan.

Adults aged 85–89 years from the Kawasaki Aging and Wellbeing Project (KAWP; n  = 942) and the Tokyo Oldest-Old Survey on Total Health (TOOTH; n  = 393).

Five IC domains were individually rescaled to a 0–100 scale and averaged, with higher scores indicating better capacity. An exploratory cardio-renal measure comprising N-terminal pro-B-type natriuretic peptide and cystatin C was entered alongside the IC score. The outcomes were incident disability (long-term care insurance certification in KAWP; activities of daily living dependency in TOOTH) and all-cause mortality. Cox proportional hazards and Fine-Gray competing-risk models were adjusted for age, sex, education, smoking status, comorbidity burden, and body mass index.

In KAWP, each 10-point increment in the IC score was associated with lower risks of care level 1 (adjusted hazard ratio [HR] 0.72, 95% CI 0.66–0.79) and mortality (HR 0.78, 0.68–0.90). In TOOTH, associations were comparable for mortality (HR 0.74, 0.60–0.91) and disability or death (HR 0.78, 0.67–0.91). The association with disability was no longer evident when death was treated as a competing event. The cardio-renal measure provided modest but consistent incremental information for mortality.

Higher IC was associated with lower risks of disability and mortality among octogenarians across two independent Japanese cohorts. These findings suggest that IC captures shared vulnerability to disability and death and may serve as a population-level marker of vulnerability rather than as a tool for individual-level risk stratification.