Although asymmetric or unilateral optic disk swelling is unusual in patients with increased intracranial pressure, we treated three such patients. Two (a 31-year-old man and a 31-year-old woman, both of whom were overweight) had benign intracranial hypertension. The third patient (a 64-year-old man) had a posterior fossa mass that was later discovered to be an anaplastic astrocytoma. Thus, unilateral optic disk edema does not exclude intracranial abnormalities. The papilledema may be unilateral rather than bilateral because of an anomaly of the optic nerve sheath or a subarachnoid hemorrhage.