PURPOSE: To access the usefulness of magnetic resonance(MR) in the evaluation of orbital blowout fracture. MATERIALS AND METHODS: Fourteen patients with orbital blow out fractures diagnosed by plain radiography(n = 8) or computed tomography(CT)(n = 6) were examined with MR. Twenty blowout fractures including six patients with two lesions were presented and the fracture site of the orbit was the medial wall in 11 cases and the orbital floor innine cases. MR images in nine cases of six patients could be compared with CT scans. On MR images, were trospectively evaluated the presence of fracture, herniation of orbital fat, abnormality of extraocular muscle, intraorbital hematoma and intrasinus hemorrhage. In nine cases we also compared these findings with CT scans. RESULTS: Eighteen orbital blowout fractures with fat herniation could be diagnosed on MR images. In only nine of these 18 cases, the fracture itself could be seen on MR images as a disruption of the orbital wall, which produce a signal void. But two blow out fractures without orbital fat herniation, seen on CT, were not detected on MRimages. Twenty abnormalities of extraocular muscle in 18 cases were depicted on MR images. There were nosignificant differences between MR and CT in the evaluation of orbital fat herniation and extraocular muscle abnormality ; however, in one case oblique sagittal MR images provided more useful information about the status ofthe inferior rectus muscle. Intraorbital hematoma was detected by MR in three patients. We were able to establish the presence of intraorbital hematoma by using the different signal intensities from fat or muscle on T1- and/or T2-weighted images. MR was found to be superior to CT in one case of intraorbital hematoma because of the iso-density of hematoma compared to muscle. MR also detected intrasinus hemorrhage which on T1-weighted images revealed high signal intensity in ten cases. MR was more valuable in demonstrating intrasinus hemorrhage in onecase where a CT scan showed only hypodense fluid. CONCLUSION: MR is a useful diagnostic modality in the evaluation of orbital blowout fracture with orbital fat herniation. Compared with CT, it also provides more valuable information about associated extraocular muscle abnormality, subacute hemorrhage in the orbit and paranasal sinus.