PURPOSE: The purpose of this study was to determine the prevalence of heterotopic ossification following knee dislocation. TYPE OF STUDY: Prospective clinical evaluation and a retrospective chart review. METHODS: This study evaluated 57 knees in 55 patients who sustained high-energy blunt trauma with resultant knee dislocations. Radiographs were reviewed by 2 of the authors (J.P.S., T.C.W.), and the incidence of heterotopic ossification (HO) was documented. Additionally, patients were classified regarding the degree of HO on a scale from 0 to 4. One is punctate calcification, 2 is HO involving less than 50% of the joint space; 3 is HO involving more than 50%; and 4 is ankylosis of the joint. RESULTS: Thirteen patients with 15 knee dislocations developed HO. The incidence of HO was 26%. Seven knees demonstrated severe HO (grade 3 or 4) which represented an incidence of 12% of all knee dislocations. Injury severity score for both groups was 18, documenting that the patients in this study represent multiple trauma patients. There was no significant difference in the incidence of HO based on mechanism of injury with the current number enrolled in the study. However, 60% (3 of 5) of patients involved in a motor vehicle versus pedestrian accident developed HO. There was a significant increase in the incidence of arthrofibrosis in patients with severe HO (P <.05). Patients with significant HO had a mean flexion of 97 degrees, compared with flexion of 117 degrees in patients with no or mild HO. This difference was borderline significant (P =.058). There was no difference between the groups in mean extension. There was a significant increase in knee HO in patients with HO at another anatomic site (P =.01). CONCLUSIONS: HO is a common problem following knee dislocation. Of the 7 knees with severe HO, 5 developed HO medially, 4 developed HO posteriorly, 3 developed HO laterally, and only 1 had involvement anteriorly. A similar distribution was present in the patients with mild HO, with posterior and medial ossification being the most common. Five of the 7 severe HO cases involved at least 3 of the 4 sides (anterior, posterior, medial, or lateral) of the knee. Patients were evaluated for the presence of head injury and any relationship to the development of HO around the knee. There was no increased incidence of HO around the knee in our 10 patients with severe head injuries when compared with those with no head injury. There was also no increased incidence of HO in knee dislocations associated with periarticular fractures.