OBJECTIVE: The purpose of this study was to test the efficacy of sonography in the diagnosis of rupture of the anterior cruciate ligament in the setting of a recent traumatic hemarthrosis. SUBJECTS AND METHODS: Sonography was prospectively performed in 37 patients with a recent traumatic hemarthrosis of the knee, no bone abnormality seen on plain radiographs, and no history of a previous knee injury. The presence of a hypoechoic collection along the lateral wall of the femoral intercondylar notch was interpreted as a hematoma at the femoral attachment of the anterior cruciate ligament. Arthroscopy was subsequently performed in 30 patients. The findings of three diagnostic techniques (sonography, MR imaging, and arthroscopy) were compared. RESULTS: The sonographic findings were confirmed by MR imaging and arthroscopy in 34 of the 37 patients. For the three false-negative results, sonographic findings were abnormal but equivocal in two cases and were reported as negative. The technique was therefore 91% sensitive and 100% specific. The positive predictive value was 100%. The negative predictive value was 63%. CONCLUSION: Sonography is a useful and inexpensive method of detecting the presence of rupture of the anterior cruciate ligament in the clinical setting of a traumatic hemarthrosis.