The aim of this study was to evaluate the Clinical Dementia Rating Scale sum of the boxes (CDR-SB) diagnostic validity in detecting and staging cognitive impairment/dementia in a sample of Brazilian patients with amnestic mild cognitive impairment (aMCI), Alzheimer disease (AD), and vascular dementia (VD), of low educational attainment. Data were obtained from the Dementia Clinic of Hospital de Clínicas de Porto Alegre database and included 407 participants (115 healthy elderly, 41 aMCI, 165 AD, and 86 VD). Receiver operating characteristic curves were generated to detect best CDR-SB cutoffs. Average education was 4 years. A CDR-SB cutoff ≥0.5 was obtained to correctly identify MCI from normal controls (sensitivity of 100% and specificity of 98.3%). The cutoff ≥4.5 correctly identified aMCI from dementia altogether or separately (AD and VD) (sensitivity of 96.4% and specificity of 100%) in 96.9% of the individuals. Similar AUC was found for ≤4 and >4 years of education. The CDR-SB showed good clinical validity to detect and classify severity of cognitive impairment Brazilian patients with low educational attainment. Findings were similar to the original study carried out with higher educated individuals.