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To determine the magnitude of upper eyelid retraction induced by sudden darkness in normal subjects and in patients with Graves upper eyelid retraction before and after treatment with guanethidine drops.The study comprised 211 control subjects (n = 211 eyes) and 45 patients (n = 78 eyes) with Graves upper eyelid retraction. The control subjects were divided in four age groups: 0 to 1 year, 2 to 9 years, 0 to 18 years, and 19 to 61 years. Twenty-one patients with Graves upper eyelid retraction (n = 39 eyes) used guanethidine drops for 15 days. Palpebral fissure images of subjects were acquired in photopic conditions and in darkness. For both images, the distance between the mid-pupil and upper eyelid margin was measured.Darkness induced upper eyelid retraction in all subjects. The increment in the mid-pupil eyelid distance was greater in children. There was no significant difference between the magnitude of eyelid elevation of Graves patients and normal adults. Guanethidine drops did not abolish the eyelid reflex in Graves patients.Darkness provokes upper eyelid retraction in control subjects and in patients with Graves upper eyelid retraction. This effect decreases with age and does not result from sympathetic stimulation of the Müller muscle.