Background: During routine diagnosis of brain death, changes in pupil diameter in response to the stimulation of peripheral nerves are sometimes observed. For example, pupillary dilation after diagnosed brain death is described in the literature as the ciliospinal reflex. However, pupil constriction creates diagnostic doubts. Objective: The pupillometric analysis of pupil response to stimulation of the cervicothoracic spinal cord in patients with diagnosed brain death. Methods: Instrumental tests to confirm the arrest of cerebral circulation were performed in 30 adult subjects (mean age 53.5 years, range 26–75 years) with diagnosed brain death. In addition, a pupillometer was used to measure the change in pupil diameter in response to neck flexion. Intervention: Flexion of the neck and measuring the response in change of the pupil with the use of the pupillometer. Results: The change in the pupil was observed in the examined group of patients. Difference in pupil size ≥ 0.2 mm was observed in 14 cases (46%). In five cases (17%), pupil constriction was found (from 0.2 to 0.7 mm). Measurement error was +/− 0.1 mm. Conclusions: Both pupillary constriction and dilatation may occur due to a ciliospinal reflex in patients with brain death. This phenomenon needs further research in order to establish its pathophysiology.