Pupillary diameters in the affected and unaffected eyes of 327 patients with uniocular red eye were assessed during fixation of a distant target. The mean pupillary diameters were similar in the unaffected eyes in each of eight diagnostic groups, but were significantly different (F = 3.84, p less than 0.001) in the diseased eyes. With corneal abrasions (p less than 0.001), marginal keratitis (p less than 0.05), and acute anterior uveitis (p less than 0.001) the mean pupillary diameter for the affected eye was significantly smaller than that of the unaffected eye. The observed differences of pupillary diameter probably reflect the role of neuronal and autocoid mechanisms in the unilateral control of pupillary size.