Five hundred consecutive patients undergoing primary cemented knee arthroplasties were prospectively followed up and evaluated for the presence of heterotopic ossification. Of the patients studied, 95% had osteoarthritis and 5% had inflammation. A radiographic grading system was devised based on the size and location of the heterotopic bone. The overall incidence of heterotopic ossification in this group was 15%. The patients who developed HO tended to be heavier than the average and were more likely to be men. This is the most comprehensive prospective study of the incidence of HO in primary cemented knee arthroplasty. Other than a small subset of the patients (4 of 500), HO does not appear to have a major influence on the outcome of knee arthroplasty.