The ability to perform instrumental activities of daily living (IADL) is important for one's level of independence. A high incidence of limitations in IADL is seen in older adults with intellectual disabilities (ID), which is an important determinant for the amount of support one needs. The aim of this study was to assess the predictive value of physical fitness for the ability to perform IADL, over a 3-year follow-up period, in 601 older adults with ID. At baseline, an extensive physical fitness assessment was performed. In addition, professional caregivers completed the Lawton IADL scale, both at baseline and at follow-up. The average ability to perform IADL declined significantly over the 3-year follow-up period. A decline in the ability to perform IADL was seen in 44.3% of the participants. The percentage of participants being completely independent in IADL declined from 2.7% to 1.3%. Manual dexterity, balance, comfortable and fast gait speed, muscular endurance, and cardiorespiratory fitness were significant predictors for a decline in IADL after correcting for baseline IADL and personal characteristics (age, gender, level of ID, and Down syndrome). This can be interpreted as representing the predictive validity of the physical tests for a decline in IADL. This study shows that even though older adults with ID experience dependency on others due to cognitive limitations, physical fitness also is an important aspect for IADL, which stresses the importance of using physical fitness tests and physical fitness enhancing programs in the care for older adults with ID. (C) 2015 Elsevier Ltd. All rights reserved.