The aims of our study were (1) to describe the ultrasonographic findings of the injured medial collateral ligament of the knee and (2) to assess the usefulness of ultrasonography in both evaluating the presence and location of the medial collateral ligament injuries and predicting the prognoses for the patients. The study group consisted of 16 patients with the clinical diagnosis of medial collateral ligament injury. We also examined 20 knee joints in 10 volunteers who had no history of injury to the knee. To predict prognoses, we divided the patients into two groups on the basis of the location of medial collateral ligament injuries. A thickened and heterogeneously hypoechoic appearance of the medial collateral ligament was considered abnormal. The normal medial collateral ligament was a thin and moderately homogeneous hypoechoic band with an average thickness of 4.3 mm (range, 3.3 to 5.6 mm) at the femoral attachment and of 2.3 mm (range, 1.3 to 3.2 mm) at the tibial attachment. All injured medial collateral ligament structures were thickened and heterogeneously hypoechoic. According to the criteria for grading of medial collateral ligament injury with stress view, 10 patients had grade III medial collateral ligament tears, three had grade II tears, and none had grade I injuries. In 15 (94%) of 19 knees, a correct diagnosis could be made with ultrasonography. Ultrasonography is useful in evaluating isolated medial collateral ligament injuries and in predicting patient outcome on the basis of the location of the medial collateral ligament injuries.