Clinical, arthrographic, and arthroscopic findings in 53 patients with acutely torn anterior cruciate ligaments (ACLs) were documented. Arthroscopy and instability tests under anesthesia were performed on all patients within 2 weeks after the initial injury. Twenty-three patients complained of extension blocks, and localized tenderness on the medial side was revealed in 26 patients at the initial examination. Aspiration from joints exhibited hemarthrosis in 52 patients. Arthroscopy revealed ACL ruptures in all patients. Four Segond's fractures, 26 meniscus tears (8 medial and 18 lateral), 1 osteochondral fracture, and 19 medial collateral ligament ruptures were revealed. Arthroscopy detected only 1 of the 5 ruptures of the posteromedial corner of the medial meniscus, which were noted on arthrography. Three ACL stumps were protruding among the femorotibial joint, which seemed to be restricting full extension. Statistical analysis showed that tenderness on the medial side was not revealed more frequently in knees with medial collateral ligament injuries than in the others. The volume of aspirated fluids in knees with no leakage in arthrography significantly increased over those with leakages (p < 0.05). Diagnosis of ACL injuries should be completed by clinical, arthrographic, and arthroscopic examinations.