A comparison was made of Down Syndrome and other mentally retarded young adults in community work and residential settings for differences in cognitive profile across age groups using the Wechsler Intelligence Scale for Children. Evidence is of a significant erosion of test scores, especially performance scale scores for the non-DS mentally retarded subjects from the third to fourth decades of life. The Down Syndrome cohort remained relatively stable for cognitive profile over time with the notable exception of the Block Design subtest which declined significantly over the two decades; and in contrast with the control sample. Further investigation is suggested in the event that the Block Design subtest has potential as a diagnostic 'marker' for the early identification of DS individuals likely to be most susceptible to premature Alzheimer's-like senile dementia.