The clinical features of 63 patients with Duane syndrome are described in this report. All data in the study were gathered retrospectively except for measurements of vertical palpebral fissure and axial eye length, which were investigated prospectively. The following new aspects of the syndrome are emphasized: 1) In most cases with heterotropia, the angle of deviation was not large (< or = 25 prism diopters). 2) Visual acuity of 0.66 (20/30) or better was recorded in 94% of the affected eyes. 3) Smaller palpebral fissure (by 1 mm or more) of the affected eye was measured in primary position among 9 of 40 (22%) unilateral cases. 4) The axial length of the affected eye (mean, 22.8 +/- 0.6 mm) was not significantly different from the uninvolved eye. 5) Two cases of heterochromia iridis (3.2%) were found among the Duane syndrome patients. 6) A 4.8% prevalence of high-tone hearing loss was detected, in addition to 4.8% of sensorineural deafness.