The assessment of activities of daily living is a central procedure in the diagnosis of dementia. Few instruments in the field allow for early detection of functional decline because the items they use refer mainly to basic activities of daily living (BADL), which do not become compromised until later in the disease process. The Interview for Deterioration of Daily Living in Dementia (IDDD) may be a valuable tool for early detection of functional decline because it includes, apart from a BADL subscale, another subscale containing a variety of instrumental activities of daily living (LADL), which are the first to be affected in dementing processes. We present an adaptation and validation of the IDDD for Spanish-speaking communities (S-IDDD). A total of 254 control subjects (CONT), 86 patients with mild memory/cognitive impairment with no dementia (CIND), and 111 patients diagnosed with probable dementia of the Alzheimer type (DAT) participated in this project. IDDD total scores (mean and SD) were as follow: CONT: 33.1 (0.4); CIND: 35.2 (3.4); DAT: 54.3 (18.6). The present validation showed no sociodemographic effects on the IDDD total scores. The IDDD demonstrated great internal consistency (alpha = .985) and reproducibility (intraclass correlation coefficient = .94). Correlations were high (r = .81; p < .1) when they took into account the whole sample, but decreased significantly when the groups were separated by pathologic condition. The scale showed significant differences between DAT versus CIND and CONT. The IADL subscale differentiated all three groups, which makes it extremely valuable for early detection of functional decline. The present study shows that the S-IDDD is a reliable adaptation of the original IDDD scale and may be used successfully in Spanish populations for staging and follow-up of subjects with dementia.