ObjectivesTo analyze the relationship between frailty and survival in older people with intellectual disabilities (IDs). DesignPopulation-based longitudinal observational study. SettingThree Dutch care provider services. ParticipantsIndividuals with borderline to profound ID aged 50 and older (N=982). MeasurementsA frailty index (FI) including 51 health-related deficits was used to measure frailty. Mean follow-up was 3.3years. The Cox proportional hazards model was used to evaluate the independent effect of frailty on survival. The discriminative ability of the FI was measured using a receiver operating characteristic (ROC) curve. ResultsGreater FI values were associated with greater risk of death, independent of sex, age, level of ID, and Down syndrome. There was a nonlinear increase in risk with increasing FI value. For example, mortality risk was 2.17 times as great (95% confidence interval (CI)=0.95-4.95) for vulnerable individuals (FI 0.20-0.29) and 19.5 (95% CI=9.13-41.8) times as great for moderately frail individuals (FI 0.40-0.49) as for relatively fit individuals (FI <0.20). The area under the ROC curve for 3-year survival was 0.78. ConclusionAlthough the predictive validity of the FI should be further determined, it was strongly associated with 3-year mortality. Care providers working with people with ID should be able to recognize frail clients and act inan early stage to stop or prevent further decline.