Introduction: The AD8 is a brief informant-based questionnaire that may also be self-administered, and which aids in identifying cognitive impairment (CI). Our goal is to assess the diagnostic accuracy (DA) of a Spanish version of that questionnaire. Material and methods: A cross-sectional study of a clinical sample of patient/informant dyads including 330 subjects with suspected CI or dementia (DEM) and 71 controls, was conducted. We evaluated internal consistency (Cronbach's alpha) and validity (partial correlations with GDS stage, Fototest results and functional index measure [FIM]). We assessed DA for CI vs no CI (GDS stage 3–4) using the area under the ROC curve (AUC), and the cut-off with the highest Youden index was determined to be optimal. Results: In the sample, 105 subjects had no CI, 99 had CI without DEM and 203 had DEM. Internal consistency was high (α 0.90, 95% confidence interval: 0.89–0.92), as were correlations with the GDS score (r = 0.72, P