Eight cadaveric lower extremities were examined by three experienced knee surgeons in blinded fashion. The knees were examined with intact anterior cruciate ligaments, sectioned anteromedial bundles, and completely sectioned anterior cruciate ligaments to evaluate detectable laxity changes. Lachman, anterior drawer, lateral pivot shift, and KT-1000 arthrometer testing were performed. Optimized biplanar radiography using a defined spatial coordinate reference system was performed with a 30-pound anterior force at 30 degrees of flexion to confirm clinical findings. Physical examination and arthrometer testing detected no difference between intact and partially sectioned anterior cruciate ligaments; these ligaments were significantly different than completely sectioned ligaments, with the Lachman test being the most sensitive. Despite consistent clinical detection of complete sectioning of the anterior cruciate ligament by both physical examination and arthrometer testing, neither method proved accurate in the diagnosis of isolated tears of the anteromedial bundle, but both did show that partially sectioned anterior cruciate ligament closely resembled intact ligament and differed significantly from completely sectioned ligament, as confirmed by radiologic data. Clinically diagnosed "partial tear" is likely to be complete rupture of the anterior cruciate ligament. Historically, clinically diagnosed partial tears of the anterior cruciate ligament have tended to "progress" to symptomatic instability. Our data imply these patients may have had functionally incompetent ligaments from time of injury and, in fact, were demonstrating the expected natural history of an anterior cruciate ligament-deficient knee.