Introduction: To improve the detection of changes in Alzheimer's disease (AD), we designed the cognitive-functional composite (CFC). As a first validation step, we investigated its test–retest reliability and feasibility of use. Methods: We performed a test–retest study with 2–3 weeks between assessments, including patients with mild cognitive impairment (MCI) or mild AD dementia and cognitively healthy participants. We calculated intraclass correlation coefficients (ICCs) type absolute agreement for all CFC measures and compared baseline and retest scores using paired-samples t-tests. We evaluated feasibility by interviewing participants. Results: Forty-three patients (40% female, mean age = 69.9) and 30 controls (50% female, mean age = 65) were included. Subtest intraclass correlation coefficients ranged from.70 to.96. We found negligible improvements after retesting on only two subtests. Overall, patients perceived the administration of the CFC as feasible. Discussion: The CFC is a stable and feasible measure in MCI and mild AD dementia, and thereby meets important quality metrics for clinically meaningful outcome measures.