To determine the test–retest reliability and criterion validity of self-reported function in mobility and instrumental activities of daily living (IADL) in older adults, a convenience sample of 70 subjects (72.9 ± 6.6 yr, 34 male) was split into able and disabled groups based on baseline assessment and into consistently able, consistently disabled, and inconsistent based on repeat assessments over 2 weeks. The criterion validities of the self-reported measures of mobility domain and IADL-physical subdomain were assessed with concurrent baseline measures of 4 mobility performances, and that of the self-reported measure of IADL-cognitive subdomain, with the Mini-Mental State Examination. Test–retest reliability was moderate for the mobility, IADL-physical, and IADL-cognitive subdomains (κ = .51–.66). Those who reported being able at baseline also performed better on physical- and cognitive-performance tests. Those with variable performance between test occasions tended to report inconsistently on repeat measures in mobility and IADL-cognitive, suggesting fluctuations in physical and cognitive performance.