Twenty-three patients who had undergone unilateral patellectomy were tested using the Cybex II isokinetic dynamometer 9 years postoperatively. The results were compared with the performance of the uninvolved joint. There was a good correlation between loss of the quadriceps muscle function and loss of the hamstrings muscle function at 60 deg/s (R = 0.7, P < 0.001). Patients who showed a loss of quadriceps function of less than 40% also maintained good hamstrings function. A good or excellent functional result could be expected in these patients. If the loss of quadriceps function was more than 40%, a proportional loss of flexion torque was seen, indicating a functional impairment of the knee joint muscles not solely attributable to the loss of the lever arm. Furthermore, all patients with a loss of peak flexion torque of more than 30% showed an unsatisfactory clinical result. The evaluation of the hamstrings muscles by measuring the peak flexion moment at 60 deg/s can therefore be used as a preoperative assessment and as a guideline for rehabilitation after patellectomy.