Spontaneous rupture of the quadriceps tendon has heen rarely reported. We experienced four knees of 3 patients. Two patients with chronic renal failure(CRF) had been treated with peritoneal dialysis and the other patient was healthy without significant medical problem. In the CRF patients, laboratory studies were coincided with secondary hyperparathyoidism. Radiologic and operative findings suggested that the repeated minor trauma caused avulsion fracture of the bone near the insertion site of the tendon. Histologic findings revealed no elastosis of the tendon and no deposit of amyloid in the quadriceps tendon. In the non-CRF case, there was normal lahoratory data, and the rupture occurred at the musculotendinous junction, suggesting sudden contracture of the muscle causing failure without trauma history. All cases were treated successfully with open repair of the quadriceps tendon. It could be summarized that the site of spontaneous rupture in the CRF and normal patient were different.