From a review of 186 patients with ‘typical knee sprains’ in two separate studies conducted at the University of Cincinnati Sports Medicine Institute (one retrospective and the second one a prospective evaluation by arthroscopy and examination under anesthesia) the actual diagnosis was compared to the pertinent historical and clinical findings. The following conclusions were reached: 1) The commonly encountered ‘mild knee sprain’ may be a serious knee injury which is easily underestimated. 2) Traumatic hemarthrosis indicates a serious knee injury which in our studies revealed significant intraarticular injury in greater than 90% of the patients. The decision to use arthroscopy, and/or examination under anesthesia should be based on the patient's age, health, and activity level, and the treatment methods available to the examining physician. 3) If an examining physician conducts a thorough historical and clinical evaluation with special emphasis on ligamentous instability and/or acute hemarthrosis followed by examination under anesthesia and arthroscopy when indicated, an accurate diagnosis can be made more than 95% of the time, with the subsequent determination of a rational treatment program.In the prospective study, a surprising 72% showed disruption of the anterior cruciate ligament. In addition, associated injuries included meniscal tears (62%), osteochondral fractures or fissures (20%), and other significant ligament disruptions (20%). The initial clinical exam alone consistently underestimated the extent of the injury, providing inadequate data for treatment decisions.We wish to emphasize that a joint effusion within 24 hours of knee injury, no matter how trivial, usually represents a hemarthrosis implying a serious injury and therefore possible need for further diagnostic tests beyond the initial examination. Regardless of the physician's method of treatment, the patient must be advised and counseled as to the potential seriousness of any knee injury resulting in an acute hemarthrosis.