This study was done to evaluate the magnetic resonance imaging (MRI) findings in acute traumatic effusions of the knee without ligament instability or radiographic abnormality other than the effusion. On reviewing the charts of 927 patients who had MRI of the knee, we found 114 patients who met the following criteria for inclusion in this study: (1) acute effusion by examination or history, (2) ligament stability on examination, and (3) normal findings on radiographs (excluding effusion). The total of 144 pathologic diagnoses made by MRI included bone injury (53%), disruption of the anterior cruciate ligament (19%), sprain of the medial collateral ligament (12%), medial meniscus tear (11%), lateral meniscus tear (5%), and rupture of the posterior cruciate ligament ( < 1%). Eight studies (7%) were read as normal. These MRI data are significantly different from data of previous studies using arthroscopy to evaluate acute knee injuries without ligament instability. The findings more accurately reflect the injury pattern seen in this setting because MRI can show both bone and soft tissue injury. From this study, we conclude that bone injury is the most common cause of acute effusion in the patient with ligament stability and normal radiographs.