Introduction This study aimed to investigate the sensitivity to change over time of a new informant-based instrument to assess instrumental activities of daily living (IADL) during the course of dementia: the Amsterdam IADL Questionnaire© (A-IADL-Q). Methods Participants (n = 102) were patients and their informants who visited the Alzheimer Center of the VU University Medical Center. Linear mixed models with random effects were used to relate longitudinal change on the A-IADL-Q to diagnosis and to longitudinal change in cognitive measures. Results We found longitudinal change on the A-IADL-Q to differ between diagnosis (P = .003), with dementia patients showing the fastest rate of decline (P < .001). In addition, we found longitudinal change on the A-IADL-Q to be related to longitudinal change in cognitive measures (global cognition: P < .001; memory: P = .024; executive functioning: P = .028). Discussion Findings indicate the A-IADL-Q is sensitive to change over time in IADL functioning and can be used in evaluating treatment effects and assessing individual disease progress.