Monocular eye closure in sunlight is often noted in patients with intermittent exotropia. Neither the reason for eye closure nor its frequency of occurrence in other populations is known. We questioned 147 consecutive nonoperated patients (and/or their parents) ages 1-45 years, 93 with strabismus with onset prior to 7 years of age and 54 without strabismus, for a history of closing one eye in sunlight. Monocular eye closure in sunlight was reported most frequently by patients with intermittent exotropia but was also reported by those with other forms of strabismus as well as by nonstrabismic normals. Cooperative subjects were studied further to determine the cause for eye closure. Each subject was exposed to a source of intense illumination while fixating a target. All who closed one eye under these conditions reported photalgia associated with the high light intensity, most noting less discomfort after closing one eye. None reported diplopia prior to eye closure. Monocular and binocular photophobia thresholds were then measured for each subject. The mean monocular photophobia threshold was significantly higher than the binocular threshold. In addition, binocular photophobia thresholds were significantly lower in those reporting eye closure compared with those who did not. Monocular eye closure in sunlight is a mechanism used to reduce photophobia and is not related to avoidance of diplopia.