Background: Previous research with earlier versions of the WISC and WAIS has demonstrated that when administered to people who have intellectual disabilities, the WAIS produced higher IQ scores than the WISC. The aim of this study was to examine whether these differences still exist. A comparison of the Wechsler Adult Intelligence Scale-Third Edition (WAIS-III) with the Wechsler Intelligence Scale for Children-Fourth Edition (WISC-IV) was conducted with individuals who were 16 years old and receiving special education. Materials and Methods: All participants completed the WAIS-III (UK) and WISC-IV (UK). The order of administration was counterbalanced; the mean Full Scale IQ and Index scores on the WAIS-III and WISC-IV were compared. Results: The WAIS-III mean Full Scale IQ was 11.82 points higher than the mean Full Scale IQ score on the WISC-IV. Significant differences were also found between the Verbal Comprehension Index, Perceptual Reasoning/Organization Index and Processing Speed Index on the WAIS-III and WISC-IV, all with the WAIS-III scoring higher. Conclusions: The findings suggest that the WAIS-III produces higher scores than the WISC-IV in people with intellectual disabilities. This has implications for definitions of intellectual disability and suggests that Psychologists should be cautious when interpreting and reporting IQ scores on the WAIS-III and WISC-IV.