The goal of the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) is “to provide a standardized, objective approach to evaluating medical impairment,” resulting in reliable, reproducible impairment ratings with high levels of interrater consistency. The authors reviewed 6233 impairment ratings that took place between July 2006 and January 2010 that reflected 11991 ratable diagnoses and found poor interrater reliability (78% disagreement rate). A previous study conducted in 2005 was published in The Guides Newsletter (May/June and July/August 2006 issues) and evaluated 2100 cases for impairment rating review and found that 80% of ratings resulted in different outcomes when reviewed by an expert reviewer. The current study found an average difference of 10.0% whole person permanent impairment (WPI) between the original WPI ratings calculated with the fifth edition and the revised ratings after expert review; the 2005 study found a similar difference, 9.9% WPI. The reasons for poor interrater reliability with fifth edition ratings are many and include inaccurate clinical and causation analysis, failure to use the AMA Guides appropriately, and bias. The error rate in this study was lowest for patients in Hawaii, where the AMA Guides, Fifth Edition, has been used since 2001, only a relatively small number of authors who have been trained in the use of the AMA Guides perform the evaluations, there is no systematic coaching by attorneys about how to use the AMA Guides, and impairment ratings are routinely reviewed to determine their accuracy.