The present study examined the validity of the Severe Impairment Battery (SIB) as a tool for measuring cognitive decline in clients with Down syndrome. Two groups participated: 10 clients who showed behavioural decline over at least a 2 year period as measured by the Vineland Adaptive Behaviour Scales, and 14 clients who showed no decline on this measure over the same period. No differences were found between the two groups in relation to health or life factors which may have impacted on functional and cognitive decline. The deteriorating group were found to be significantly older than the non-deteriorating group. The comparison of the SIB scores indicated that the deteriorating group showed a significant decline between baseline and 12 months and baseline and 24 months on the orientation factor. By contrast, for the non-deteriorating group, significant increases were found for praxis, orientating to name and total scores.