Background: This study investigated the application of a newly developed neuropsychological assessment, the Wolfenbütteler Dementia Test for Individuals with Intellectual Disabilities (WDTIM) in combination with the Dementia Screening Questionnaire for Individuals with Intellectual Disabilities (DSQIID). Methods: The instruments were evaluated in a prospective 2-year follow-up study. A total of 102 people with an intellectual disability were assessed at 6-month intervals. Data were analysed using qualitative and statistical analyses. Results: Four groups of individuals emerged from the analysis: (1) confirmed suspicion, (2) no suspicion, (3) questionable suspicion and (4) early suspicion. Significant differences were found between groups 1 and 2. The WDTIM could be administered to 90%-100% of all participants exhibiting mild-to-moderate intellectual disability and to 50% with severe intellectual disability. Conclusions: The WDTIM was shown to have good applicability to people with mild-to-moderate intellectual disability and to be appropriate for detecting cognitive changes. Using the two instruments in combination achieved greater accuracy in reinforcing a dementia suspicion than did using the DSQIID alone.