Impairment apportionment refers to the distribution or allotment of impairment to different factors such as pre-existing injury, illness, or impairment. The AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) provides instructions about calculating impairment ratings and advises concurrent consultation of specific state laws (eg, the state of California apportions disability differently than does the Longshore and Harbor Workers Act. A case example highlights some of the complexities of cervical spine apportionment. A 55-year-old man sustained a work-related cervical spine injury that aggravated pre-existing cervical pathology. The patient underwent anterior cervical discectomy and decompression, foraminotomies, C3-4 and C4-5, and a two-level anterior cervical interbody arthrodesis; the postoperative diagnosis was two level disc herniation, stenosis, and disc degeneration. The article discusses the Diagnosis-related estimate (DRE) vs the range-of-motion method (ROM). The DRE method is most widely used in the AMA Guides, but in certain specified situations, including a multilevel fusion such as that in the clinical example, the ROM method must be used. Because of the significant difference in values derived from the DRE and ROM methods, evaluators must be careful to apportion impairment in accordance with the method that served as the basis for the final rating (ie, the ROM method in this example). Apportionment becomes more complex when one considers the etiology of the underlying disease.