Heterotopic ossification is a rare complication following primary total knee arthroplasty and may be symptomatic if massive enough. The authors retrospectively reviewed 158 primary total knee arthroplasties from 1985 to 1989 and found 6 cases (3.8%) of heterotopic ossification. Patients were graded before and after surgery according to the Hospital for Special Surgery total knee arthroplasty score and their histories were reviewed for the presence of recognized risk factors for heterotopic ossification and whether a manipulation under anesthesia was performed. This report describes the incidence of, appearance of, and clinical risk factors for heterotopic ossification following primary total knee arthroplasty in this series. A radiographic grading system is proposed.