A prospective, arthroscopic controlled study was performed to evaluate the usefulness of manual sonometry of the knee in the diagnosis of acute rupture of the anterior cruciate ligament (ACL). In 58 patients with an acute knee injury, a maximum of 30 days elapsed between accident and clinical examination (Lachmantest, pivot-shift/McIntosh, 90°anterior translation), manual sonometry and functional X-ray examination. Afterwards all patients were examined by arthroscopy. Nine patients showed a partial, 38 patients a total rupture of the ACL. If the ACL was completely ruptured, the average difference in anterior translation between the contralateral and the injured knee was 3.3 mm (p ≤ 0.001). Statistical analysis showed high sensitivity (85%), specificity (91%) and positive predictive value (98%) for manual sonometry. Clinical examination, apparative sonometry and functional X-ray were, overall, less effective than manual sonometry in detecting ligament rupture. Manual sonometry proved to be a good and practicable method for assessing acute rupture of the ACL.