Objective: Clinical observations of large discrepancies in individual intelligence quotient (IQ) scores calculated using Canadian vs. American norms for the WAIS-IV have raised questions about the appropriateness of the Canadian normative system for use with a clinical population in Canada. The purpose of this study was to compare the interpretive effects of applying the American and Canadian normative systems in a clinical sample. Method: We analyzed archival data from 338 Canadian individuals who received routine neuropsychological assessment between 2008 and 2010. Differences between Canadian and American standard scores for the Wechsler Adult Intelligence Scale–IV (WAIS-IV) data were determined using a Multivariate Analysis of Variance (MANOVA) and three mixed model Analyses of Variance (ANOVAs) were used to further assess the pattern of differences across Canadian and American normed standardized scores. Results: Full Scale IQ (FSIQ) and index scores calculated using the Canadian normative system were systematically lower than those calculated using the American system. The largest differences in FSIQ were obtained for individuals classified as having Extremely Low or Borderline intellectual functioning, and for individuals below the age of 45. In our sample, the choice to use the Canadian rather than the American normative system resulted in clinically different classification of greater impairment in intellectual ability for 52.8% of individual patients. Conclusion(s): Our findings underscore the need for caution when choosing a normative system with which to interpret results of the WAIS-IV in the context of a neuropsychological test battery in Canada.