Several Canadian provinces use the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) to adjudicate workers’ compensation claims, and the Province of Ontario uses the AMA Guides, Fourth Edition, to adjudicate motor vehicle accident personal injury claims. This article focuses on controversies that have arisen in Ontario regarding how the AMA Guides is applied and shows some of the challenges that occur in quantifying psychological impairment. In 2004, the Ontario Superior Court found in the Desbiens v. Mordini trial that the AMA Guides did not provide any direct methodology for estimating percentage impairment in this unique circumstance that involved pre-existing paraplegia and subsequent dramatic loss of residual functions. The judge found that, using the information available, a whole person impairment (WPI) score of 40% could be derived, but Ontario requires a minimum 55% WPI before an individual qualifies for catastrophic impairment benefits. In view of the individual's circumstances and a psychologist's recommendation, the judge awarded an additional 25% WPI. The Ontario model has been interpreted to allow subjective complaints (symptoms) to be included in the impairment evaluation process, but this approach eliminates any expectation of objectivity. If a judicial system aims to force impairment percentages onto a situation that in fact does not warrant such ratings, it should not do so by an inappropriate application of the AMA Guides.