Ninety-eight consecutive primary total knee arthroplasties (TKAs) in 70 patients were retrospectively evaluated for heterotopic ossification (HO). A radiographic classification was devised based on the extent and location of the ectopic bone. Twenty-five knees (26%) in 19 patients developed HO. Eight of 11 patients (73%) with preexisting heterotopic bone at other sites developed HO in the index knee. Multivariate analysis demonstrated that advanced HO was associated with restricted knee motion. Eight knees with advanced HO had a mean 14 degrees decrease in postoperative, as compared with preoperative, knee flexion (P < .05). For all patients with HO, mean lumbar spine bone mineral density (BMD) was significantly elevated compared with a matched control group not developing HO (P < .05). Heterotopic ossification following primary TKA correlates with a limitation of postoperative knee flexion and is predicted by increased lumbar BMD. Preoperative measurement of spinal BMD may identify those patients at risk for HO and allow for the institution of preoperative prophylaxis and modification of postoperative rehabilitation to optimize functional outcome following TKA.