Background: Measuring impairments in "instrumental activities of daily living" (IADL) is important in dementia, but challenging due to the lack of reliable and valid instruments. We recently developed the Amsterdam Instrumental Activities of Daily Living Questionnaire (A-IADL-Q; note 1). We aim to investigate the diagnostic accuracy of the A-IADL-Q for dementia in a memory clinic setting. Methods: Patients visiting the Alzheimer Center of the VU University Medical Center with their informants between 2009 and 2011 were included (N = 278). Diagnoses were established in a multidisciplinary consensus meeting, independent of the A-IADL-Q scores. An optimal A-IADL-Q cutoff point was determined, and sensitivity and specificity were calculated. Area under the curves (AUCs) were compared between A-IADL-Q and "disability assessment of dementia" (DAD). The additional diagnostic value of the A-IADL-Q to Mini-Mental State Examination (MMSE) was examined using logistic regression analyses. Results: Dementia prevalence was 50.5%. Overall diagnostic accuracy based on the AUC was 0.75 (95% confidence interval [CI]: 0.70-0.81) for the A-IADL-Q and 0.70 (95% CI: 0.63-0.77) for the DAD, which did not differ significantly. The optimal cutoff score for the A-IADL-Q was 51.4, resulting in sensitivity of 0.74 and specificity of 0.64. Combining the A-IADL-Q with the MMSE improved specificity (0.94), with a decline in sensitivity (0.55). Logistic regression models showed that adding A-IADL-Q improved the diagnostic accuracy (Z = 2.55, P = .011), whereas the DAD did not. Conclusions: In this study, we showed a fair diagnostic accuracy for A-IADL-Q and an additional value in the diagnosis of dementia. These results support the role of A-IADLQ as a valuable diagnostic tool. © 2013 The Author(s).