ABSTRACTAs computerised tomography is now accepted as a method of choice for the investigation of suspected orbital masses, measurement of the degree of proptosis (exophthalmos) should be useful in assessing response to therapy.Review of a number of orbital CT scans (GE 8800) revealed that both scanning and measuring techniques must be standardised to obtain consistent results.The following requirements must be met:The plane of the scan must be parallel to the plane passing through the optic nerve head and lens, i.e. 15 degrees cephalad to the orbitmeatal line.The eyelids must be open, with the patient looking straight ahead.Window levels must be set appropriately when the measurements are made, i.e. +500 HU for the corneal surfaces.A line between the bony lateral orbital margins is drawn on the slice containing the optic nerve head and lens, and the distance of the cornea in front of this line is measured.When the radiographic criteria are met, proptosis can be measured with accuracy of 0.8 mm.