The CAMDEX-DS is an instrument to diagnose Alzheimer's disease (AD) in Down syndrome consisting of an informant interview and a cognitive test battery (CAMCOG-DS). Measurement properties of the German CAMDEX-DS were investigated. Fifty-five adults with Down syndrome (19-58 years) participated in this observational study. "Dementia" and "Alzheimer's dementia" (Alzheimer's disease) were diagnosed clinically and operationalised CAMDEX-ICD-10 criteria were applied to evaluate criterion validity. Validity and reliability of the CAMCOG-DS were analysed. Specificity of the interview was 69-93%; sensitivity 0-80% for "dementia"; and 0-20% for Alzheimer's disease. A complete CAMCOG-DS score was obtained in 85% (item difficulty 0.11-0.96). Construct validity and retest-reliability were low to moderate (τ = .04-.79), inter-rater reliability excellent (τ = .70-.89), internal consistency and selectivity acceptable to excellent. Currently, the CAMDEX-DS including the CAMCOG-DS are the outcome assessments for assessing dementia in Down syndrome with the best psychometric properties; however, revision is recommended.