The purpose of this study was to assess the diagnostic value of orbital line changes on plain radiographs in detecting blow-out fracture. 92 cases of surgically confirmed blow-out fracture were retrospectively analysed in regard to plain radiographs and CT. Anterior and posterior lamina papyracea lines (ALPL and PLPL, respectively) of the orbital medial wall as well as the posteromedial floor line (PMFL) were assessed on orbital posteroanterior projections. The inferomedial orbital line (IMOL) and the inferior wall line were assessed on Waters projections. Orbital lines on plain radiographs were compared with CT findings. Of 53 cases of lamina papyracea fracture, 47 cases showed orbital line changes on plain radiographs (sensitivity 88.7%). Changes of PLPL (41/47), IMOL (29/47), ALPL (4/47) and PMFL (2/47) were demonstrated as depression, loss, discontinuity and irregularity. In four cases showing normal radiographs, the fractures measured less than 9 mm in size on CT. All 25 cases with orbital floor fracture accompanying medial wall fracture demonstrated orbital line changes of PMFL (16/25), PLPL (14/25), ALPL (3/25) and IMOL (2/25) (sensitivity 100%). 12 of 14 cases with orbital floor fracture demonstrated changes of inferior wall line (7/12) and PMFL (6/12) (sensitivity 85.7%). Two cases demonstrated asymmetric focal soft tissue density without orbital line changes. Orbital line changes on plain radiographs corresponded well with CT findings, confirming the usefulness of plain radiographs in detecting blow-out fracture.