Hoyt BW, Roach WB, Lundy AE, et al. - This research was sought to confirm the Walter Reed classification system for reliable characterization of heterotopic ossification (HO) burden in residual limbs. The orthogonal radiographs of 100 randomly selected residual upper and lower limbs after combat-related amputation were assessed by five orthopaedic surgeon raters with varying experience levels. Scores were analyzed for interobserver and intraobserver reliability using weighted Cohen's kappa, intraclass correlation coefficient, and Krippendorff's alpha for ordinal variables. The results demonstrate that Walter Reed HO classification system is substantially reliable for use by raters with any level of orthopaedic training. In addition,  in the upper and lower extremities, it is reliable for assessment of both through bone and disarticulation type amputations. Nevertheless, in 11% of cases, a secondary read may vary, implying that this system is beneficial for understanding HO and developing interventions but may benefit from further refinement, including advanced imaging and clinical correlation.