PurposeThe tendon width of the lateral rectus muscle is known to be a useful indicator for estimation of the effect of lateral rectus recession in intermittent exotropia. This study was conducted to investigate whether the tendon width of the lateral rectus would differ according to different age groups.Patients and methodsWe studied 133 patients ranging from 0 to 51 years of age who had undergone bilateral lateral rectus (BLR) recession for the basic type of intermittent exotropia. A total of 133 patients were divided into four groups; 16 patients who were younger than 2 years old (group 1), 20 patients who were 2-5 years old (group 2), 75 patients who were 5-13 years old (group 3), and 22 patients who were older than 13 years (group 4). Under general anesthesia and before dissection of the muscle tendon from the sclera, the tendon width of the lateral rectus of both eyes near insertion was measured with calipers.ResultsThe tendon width of each group was as follows: in group 1, 7.84±0.35 mm in the right eye and 7.66±0.44 mm in the left eye; in group 2, 7.70±0.50 mm and 7.65±0.52; in group 3 8.11±0.36, 7.95±0.48. In group 4, measurements were 8.14±0.49 mm and 8.05±0.38 mm, respectively. The difference of tendon width in both eyes was statistically significant in all four groups (P<0.01) and the tendon widths of group 1 and 2 were narrower than that of group 3 and 4.ConclusionMeasurement of tendon width of the lateral rectus muscle for prediction of the effect in intermittent exotropia should be applied in patients ≥5 years of age.Eye advance online publication, 29 July 2011; doi:10.1038/eye.2011.178.