The accuracy of sonography in the diagnosis of acute rupture of the anterior cruciate ligament (ACL) was tested. Sixty-two patients with a recent traumatic haemarthrosis were examined. A haematoma at the origin of the ACL in the femoral intercondylar notch was interpreted as evidence of ligament injury. The standard of reference was arthroscopy or clinical follow-up. The sonographic findings were confirmed in 59 of 62 cases. The sensitivity was 88%, the specificity 98%, and the positive and negative predictive values 93 and 96%.