Background People with Down Syndrome are at significant risk of developing Alzheimer's disease as they get older and early assessment, diagnosis and intervention is essential. Neuro-psychological measures of cognitive functioning play an important part in the assessment process. The aim of the present study was to examine the concurrent criterion validity of the Severe Impairment Battery (SIB) when used to assess cognitive functioning in adults with Down Syndrome. Materials and Methods The study used a cross-sectional correlation design. In order to assess the concurrent criterion validity of the SIB, 37 participants with Down Syndrome completed this test, whilst caregivers completed the Dementia Questionnaire for Mentally Retarded Persons (DMR). The DMR served as the criterion measure. Results Scores on the SIB ranged from 41 to 100 (mean 78.97). Floor effects were minimal, but ceiling effects were evident. The correlation of the SIB total score with the DMR Sum of Cognitive Scores (SCS) was -0.73 indicating a good level of concurrent criterion validity. Conclusions Consistent with previous research, the SIB has good validity as a measure of cognitive functioning in people with Down Syndrome and is appropriate for use in the assessment of dementia in this population.