Between 1973 and 1985, 18 knees in 17 patients were treated for rupture of the patellar tendon after total knee arthroplasty (TKA), accounting for 0.17% of the TKAs performed at the authors' institution during the same interval. Follow-up study was from two and one-half years to four years. Four ruptures occurred in patients who had had a distal patellar realignment procedure, and one occurred after knee manipulation. Only two xenograft reconstructions and two of four staple fixation procedures were successful. After treatment of the patellar tendon rupture, four knees developed deep infection. Avoidance of this complication seems paramount because the results of treatment are discouraging. Patients at high risk at the time of the initial TKA seem to be those with limited preoperative motion in whom surgical exposure is difficult.