PURPOSE: To clarify the distribution of joint effusion, and the relationship between type of injury andamount of joint effusion seen in traumatic knee joint magnetic resonance imaging (MRI). MATERIALS AND METHODS: Weretrospectively reviewed the MR images of 400 patients with traumatic knee joint effusion. The knee joint spacewas divided into four compartments: central portion (para-ACL, para-PCL), suprapatellar pouch, posterior femoralrecess, and subpopliteal recess, and we then compared the amount and distribution of effusion. For statisticalanalysis, the chi-square test was used. RESULTS: Among 400 MRI examinations of joint effusion, 383 knees (96%)showed homogeneous low intensity on T1-weighted images, and - except for ten cases of fluid-fluid levels-homogeneous high intensity on T2-weighted images. Knee joint effusion was clearly shown to be distributed mainlyin the suprapatellar pouch (345, 86%), followed by the central posterior femoral recess, and the subpoplitealrecess (p<0.001). Extensive joint effusion was less frequently found in the normal group, but was occasionallyfound in the combined injury group (p<0.001). The relationship between amount of joint effusion and type ofinjury was statistically significant (p<0.001), except in the case of medial and lateral collateral ligamentinjury. CONCLUSIONS: The distribution of joint effusion in patients with traumatic knee disorders is a reflectionof anatomic communication, and whether the amount of joint effusion was small or large depended on the anatomicallocation and type of injury.