Background: It is complicated to diagnose dementia in persons with Down syndrome (DS). Older studies have, however, demonstrated low-frequency activity in electroencephalography (EEG) in persons with concurrent DS and Alzheimer's disease (DS-AD). The aim of this study was to examine whether it was possible to identify AD-associated changes (increased high-frequency power and decreased low-frequency power) in persons with DS-AD compared with DS. Methods: We included 21 persons with DS-AD and 16 with DS without cognitive deterioration assessed by the informant-based Dementia Screening Questionnaire in Intellectual Disability. EEG was recorded for all participants. Absolute power for each electrode and global power were calculated for all frequency bands for both eyes open and eyes closed. Results: For global power in the eyes closed condition, we found an increased global slow-frequency activity and a decreased global high-frequency activity in DS-AD compared with DS. In addition, we found a significant difference in the global alpha/delta ratio with the largest difference found for global alpha power in DS-AD compared with DS. Conclusions: In the current study, we found that changes known to be associated with AD could also be identified when comparing DS-AD with DS using quantitative EEG. In general, these findings suggest that EEG might be a useful tool in diagnosing AD in persons with DS, but larger studies are needed.