The purpose of the study was to determine the long-term healing potential of nonoperatively treated isolated sprains of the medial collateral ligament (MCL) of the knee joint. A multidimensional follow-up analysis was performed an average of nine years after injury, with 54 patients with Grade II sprains (partial tear of the MCL) and 27 patients with Grade III (complete tear) sprains. The methods used included four standardized knee scoring scales for subjective, objective, functional, and roentgenographic evaluations, as well as isokinetic and isometric strength evaluations. The outcome in Grade II sprains of the MCL was generally good, even though some residual medial laxity was common. In Grade III sprains the outcome was much worse, with a high frequency of persisting medial instability, dysfunction of the anterior cruciate ligament, muscle weakness, and posttraumatic osteoarthritis of the injured knee. Grade II sprains of the MCL respond acceptably to nonoperative treatment, whereas in Grade III sprains such treatment is not the method of choice.