OBJECTIVE The purpose of our study was to define the anatomy of the extrasynovial space that cruciate ligaments occupy by examining the pattern on MR imaging of normal fluid distribution in the joints around the cruciate ligaments and correlating this distribution with histologic analysis of synovial reflections around the cruciate ligaments. METHODS MR images of five cadaveric knees were obtained serially after larger and larger amounts of contrast material were injected into the joint space. The patterns of fluid distribution around cruciate ligaments were noted. In two other cadaveric knees, the synovial sheath around the anterior cruciate ligament was injected directly with contrast material under CT guidance. Anatomic and histologic correlation was made with findings on corresponding MR images. RESULTS The pattern of fluid distribution is bounded by the synovial reflections around cruciate ligaments. When maximum joint distention is achieved, fluid almost surrounds the cruciate ligaments. The area without fluid is a triangular space between the anterior and posterior cruciate ligaments that appears on the midsagittal image. This triangular space of the cruciate ligaments is an extrasynovial space within which both the anterior cruciate ligament and the posterior cruciate ligament reside. CONCLUSIONS The overlying synovial membrane of the cruciate ligaments does not normally allow joint fluid to enter the substance of the ligaments or the triangular space of the cruciate ligaments. Therefore, fluid collections seen on MR imaging in these extrasynovial spaces or structures likely arises from injury to the cruciate ligaments.