Post-traumatic knee injuries often deteriorate if reuptures of anterior cruciate ligaments (ACL) are not treated adequately. Further diagnostic procedures are required if pain-induced muscle contraction impairs clinical examination. The clinical relevance of sonography, however, for the diagnosis of ACL ruptures is controversial. Within 25 months, 74 patients were preoperatively evaluated clinically and by ultrasound. 58 ruptures of the ACL and 21 lesions of the collateral ligaments were confirmed by arthroscopy or arthrotomy. In 16 cases a menisceal tear was found, four patients had a luxation of the patella, and two had PCL ruptures. The sensitivity of the Lachman test was 93%, of the anterior drawer test 68% and of the pivot shift test 48%. Ultrasound diagnosis revealed 88% of all ACL ruptures (91% if patients with bony ruptures of the ACL were excluded). In 75% of ACL ruptures, a popping sensation was reported by the patients. In the case of uncertain diagnosis, examination with ultrasound could be easily performed before further operative diagnostic procedures are scheduled.