The incidence of heterotopic ossification (HO) after total knee arthroplasty (TKA) and its effect on the postoperative range of motion (ROM) were assessed in 63 primary TKAs. Twenty-five of 63 knees (39%) showed postoperative HO, most of which were found in the region anterior to the distal femur. The incidence of HO was significantly higher (P < .05) in the osteoarthritic knees than in the rheumatoid knees. Significant positive correlation was seen between the preoperative grade of osteophyte formation and the incidence of postoperative HO. The ROM in the knees with HO was not significantly different than that in the knees without HO at 1 year after surgery. Thus, we concluded that HO after TKA is a self-limiting condition and its pathogenesis is completely different from that of myositis ossificance.