Background: This study describes the validation of a new instrument measuring instrumental activities of daily living (IADL), the Amsterdam IADL Questionnaire© (a registered copyright of the Alzheimer Center of the VU University Medical Center). This informant-based tool aimed at detecting IADL problems in incipient dementia was previously found to have a high internal consistency and test-retest reliability. Methods: Patients and their informants who visited the Alzheimer Center of the VU University Medical Center were included in this study. Item response theory was used to estimate the individuals' trait levels as a measure of IADL disability. Construct validity was tested by correlating estimated trait levels with clinical and demographic measures using Pearson's or Kendall's τ correlation coefficients. Additionally, estimated trait levels between patients with and without dementia and between patients with early-and late-onset dementia were compared using independent t tests. Results: A total of 206 informants of patients completed the Amsterdam IADL Questionnaire. The correlations between estimated trait levels and other measures were in concordance with previously formed hypotheses. Patients diagnosed with dementia (n = 93) had higher estimated trait levels than patients without dementia (n = 96), Cohen's effect size, d = 1.04, t(187) = 7.1, p < 0.001. We found no differences between early-and late-onset dementia patients. Conclusions: Results suggest that the Amsterdam IADL Questionnaire is a reliable and valid instrument in the evaluation of dementia. Copyright © 2013 S. Karger AG, Basel.