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To assess the effect of simultaneous measurement of the two eyes in Hertel exophthalmometry (i.e., when both readings are taken sequentially, without removing the instrument from the orbital rims) and to find out whether simultaneous measurement would influence the final reading of each eye.In this prospective, comparative study, Hertel exophthalmometry was performed on 61 patients by two experienced observers. Only patients with no history of orbital disease were included. Three different measurements were performed on each patient by each observer. Initially, the right eye was measured separately, then the left eye was measured separately, and finally, both eyes were measured together.When the eyes were measured separately, in 30% of cases observers obtained equal readings in both eyes; in 47%, the difference between the two eyes was 1 mm; and in 23%, the difference was 2 mm or more. However, in simultaneous bilateral measurements, in 73% of cases, observers obtained equal readings in both eyes; in 24%, the difference between the two eyes was 1 mm; and only in 3% was the difference 2 mm or greater. Paired t tests showed that the mean difference between measurements of the two eyes was significantly smaller in simultaneous bilateral measurements (p < 0.001). Interobserver variation was similar for measurements taken separately on each eye and for simultaneous bilateral measurements.When both eyes were measured simultaneously with the Hertel exophthalmometer, examiners were inclined to obtain similar readings in both eyes of patients with symmetrical-appearing eyes. The possible bias arising this way may reduce the reliability of Hertel exophthalmometer in relative exophthalmometry.