In 2002, the Department of Labor and Employment, the State of Colorado, performed a study to identify changes among the Third Edition, Revised (December 1990); the Fourth Edition (June 1993); and the Fifth Edition (November 2000) of the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides). This three-part article summarizes the differences found. The study was based on a random sample of 250 cases identified by the Division of Workers’ Compensation of the State of Colorado and stratified into three broad categories: 40 upper extremity cases, 60 lower extremity cases, and 150 whole person cases. Only case records with adequate documented evidence to enable rating among all three editions of the AMA Guides were included. The average age of the examinee was 42.9 years (SD, 11.1 years, range, 18 to 71 years, 171 [68%] male); equal percentages of men and women were present in the upper extremity impairment ratings (20 men, 20 women), but lower extremity and whole person impairments occurred primarily among males (73% and 71% men, respectively). Interrater reliability was obtained from an independent expert medical review of 20% of the cases. Three figures show percentages of upper extremity, lower extremity, and whole person impairment according to the edition used; ratings generally are lower with more recent editions.