Measurement of orbital volume was conducted in 10 patients with blow-out fractures as well as in 20 controls. A bony orbital volume increase of up to 13.4+ +/- 6.0% was noted in the preoperative measurements. Proper reconstruction of the bony orbit in four patients reversed the volume change observed and achieved a satisfactory outcome. In the surgical techniques, we stress the importance of posterior floor restoration. Judicious analysis of the image and volume changes using computed tomography (CT) may offer great help in both pre- and postoperative assessments. An intimate correlation between the bony volume change and the observed degree of clinical enophthalmos has been confirmed. For a 1 mL increment in bony volume, there is a corresponding enophthalmos decrement of 1 mm on the Hertel exophthalmometer. In applying this information to clinical practice, we advocate that a bony volume change of over 8% (2 mL) usually implies a poor cosmetic outcome, thus indicating the need for surgical correction.