Under California Labor Code Section 3212.1, firefighters, forestry and fire protection workers, peace officers, and first responders receive a potentially rebuttable presumption of compensability for certain conditions, including cancer and leukemia. This case example illustrates the need for accurate clinical, causation, apportionment, and impairment assessments that are based on the facts, current science, and appropriate use of the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides). A retired California firefighter in his mid-sixties presented for an impairment evaluation for the sequelae of his colon cancer; six years earlier, he was diagnosed with stage 1 colorectal cancer and received neoadjuvant chemotherapy and radiation therapy, followed by surgical resection with a diverting colostomy and take-down surgery. He is morbidly obese (body mass index, 45) and has hypertension. He reported fecal frequency, urgency, and incontinence and stated he had problems with erectile dysfunction (ED) before his diagnosis of colon cancer, but after diagnosis and treatment the problem worsened to impotence. The author presents impairment assessments of this case example according to the AMA Guides, Fifth Edition vs the Sixth Edition. According to the fifth edition, this individual's impairment for lower gastrointestinal tract impairment is 17% whole person impairment (WPI), and this is combined with 5% WPI for ED, resulting in 21% WPI. According to the sixth edition, the lower gastrointestinal tract impairment of 14% is combined with 5% WPI for ED, leading to an 18% WPI.