Eighty-one patients with 91 partial ruptures of the patellar ligament verified by ultrasonography were treated conservatively with a standardized training program based on the principles of eccentric loading. The partial ruptures were quantified by measuring the length of the hypoechoic lesion using ultrasonography in longitudinal projection. The partial ruptures were then divided into three groups: Grade I (less than 10 mm), Grade II (10 to 20 mm), and Grade III (greater than 20 mm). The results after the training program were significantly better in Grade I ruptures than in Grades II and III. The overall need for surgery was 29.6%: highest in Grade III ruptures (38.5%) and lowest in Grade I ruptures (6.6%). The outcome of conservative management, as well as the need for surgery of partial ruptures of the patellar ligament, may to some extent be predicted by the use of ultrasonography.