Objective: It has been suggested that, as the Wechsler Adult Intelligence Scale-Third Edition (WAIS-III) and the Wechsler Intelligence Scale for Children-Third Edition (WISC-III) give a scaled score of one even if a client scores a raw score of zero, these assessments may have a hidden floor effect at low IQ levels. The study looked for indications of this in a sample of assessments that had been given for clinical and diagnostic reasons. Design: The degree to which a hidden floor effect could be present was assessed by looking at the proportion of scaled scores of one in IQ bands: 50-59, 60-69 and 70 plus and by plotting the distribution of scaled scores in these bands for both the WISC-III and WAIS-III. Method: Fifty WISC-III and 49 WAIS-III assessments were obtained from records and analysed. Results: The distribution of scaled scores in the WAIS-III was approximately normal with very few scale scores of one, suggesting that a hidden floor effect would only be a potential problem for IQs in the 40s and 50s. The WISC-III had a skewed distribution of scaled scores with more scaled scores of one than any other scaled score. Scaled scores of one were shown in all IQ levels up to 70 plus. Conclusions: There is potentially a significant floor effect on the WAIS-III at IQs in the 40s and 50s and on the WISC-III up to IQs in the 70s. There are also indications that the WISC-III has a much harder criterion for gaining a scaled score of two than the WAIS-III, resulting in it producing lower IQs.