We examined implicit and explicit memory in adults with Williams syndrome. An age-related dissociation was found; repetition priming (reflecting implicit memory) did not show change with age, but free recall (reflecting explicit memory) was markedly reduced. We also compared the performance of adults with Williams syndrome to adults with Down syndrome and those with unspecified mental retardation. A similar dissociation was observed in adults with Down syndrome but not in adults with unspecified mental retardation. An IQ-related dissociation was also found. Implicit and explicit memory, therefore, show different degrees of association with age and IQ, supporting theories of these memory processes. Results also suggest that Williams syndrome, similar to Down syndrome, may be associated with precocious aging, resulting in the loss of some cognitive abilities.