Truly unilateral papilledema is rare and poses a diagnostic problem. The authors have prospectively looked for patients with truly unilateral papilledema and found 15 patients, 10 of whom had idiopathic intracranial hypertension. Neuroimaging did not indicate a reason for the lack of swelling in the other nerve. The visual deficits and outcomes were similar to those of patients with bilateral papilledema. Although monocular papilledema is uncommon, a lumbar puncture with opening pressure measurement should be considered.