As depression is common in older people and confers significant risk for dementia, its accurate assessment is essential. The 15-item Geriatric Depression Scale (GDS-15) is a widely used assessment tool for measuring depression in aged populations, and its psychometric properties have been recently improved using Rasch analysis. However, its temporal reliability and ability to distinguish between dynamic and enduring symptoms of depression have not been examined using the appropriate methodology. Generalizability theory (G theory) is a suitable method to distinguish between enduring and dynamic symptoms of depression and to evaluate the reliability of the GDS-15 scores and sources of measurement error. We applied G theory to the longitudinal GDS-15 data of 354 participants from the Sydney Memory and Ageing Study, collected biennially over 10 years, from individuals aged 70 years and older. The GDS-15 demonstrated strong reliability and generalizability of its test scores in measuring enduring symptoms of depression (Ga = 0.82, Gr = 0.90) across the sample population and occasions, and indicated that depression scores significantly increased over time. In addition, three identified dynamic symptoms of depression, namely helplessness, hopelessness, and loss of interest in activities, did not affect the overall reliability of the GDS-15. Thus, the GDS-15 is a reliable measure for assessing enduring symptoms of depression and can be used to evaluate the efficacy of depression treatments and monitor depression levels over time in older adults. (PsycInfo Database Record (c) 2022 APA, all rights reserved).