Two patients with patellar tendon rupture and underlying systemic disease are described. One had systemic lupus erythematosus and the other had chronic renal failure and secondary hyperparathyroidism. There have been 13 other cases of patellar tendon rupture in patients with systemic disease reported in the literature and they are reviewed. Patellar tendon rupture has occurred spontaneously in more than one-half of the cases and has been associated with rupture of the contralateral extensor tendon mechanism in 87%. Although patellar tendon rupture almost always occurs at the tendinous insertion, on a rare occasion (as in one case described in this report), the tear may involve the main substance of the tendon, which is the part of the tendon with the greatest tensile strength. The relation between systemic disease and patellar tendon rupture is discussed and the clinical and radiographic findings are described.