There is currently no structured classification system to quantitate heterotopic bone formation after artificial disk replacement procedures. The purpose of this work was to develop a method of classifying heterotopic bone formation that is reliable between investigators with different levels of training and easy to remember with only five gradations of severity. One hundred one radiographs of clinical patients and 17 microradiographs from nonhuman primates having undergone various types of disk replacement were classified by seven independent reviewers. The kappa statistics were calculated for interobserver variation between the seven participants with various levels of spinal training and the intraobserver error based on two assessments made at least 2 months apart. The interobserver reliability correlation coefficient for seven raters calculated using the intraclass kappa correlation coefficient and the Kish rho was r = 0.9683 (P < 0.0001). The intraobserver reliability based on readings at two time intervals at a minimum of 2 months apart was r = 0.8949 (P = 0.01). This classification of heterotopic ossification, periannular calcification, and ectopic bone formation associated with total disk arthroplasty proved to be highly reliable and reproducible.