The relationship between age and IQ was evaluated in a cross-sectional sample of 80 individuals with Williams syndrome (17 to 52 years). The relationship between age and WAIS-R subtest scores was such that increases and decreases in raw scores occurred at a rate sufficient to maintain stability of age-corrected scaled scores, indicating a developmental trajectory similar to that of the WAIS-R normative sample. Despite stability of age-corrected scaled scores with age, increased age was related to higher Performance IQ. This disparity, which occurs during the conversion of sums of scaled scores to IQs, may be unique to the WAIS-R. Although Performance IQ increased with age, results imply that the overall IQ of an adult with Williams syndrome will likely remain stable.