Visual evoked potentials (VEPs) were elicited from 29 patients who had experienced a previous attack of acute primary angle closure glaucoma. The VEPs were shown to be abnormal in at least one of the measures (latency, amplitude, contrast threshold, or slope) in 72.4% of affected eyes, whereas only 41.4% indicated obvious optic nerve damage. It is notable that 48.1% of fellow eyes with no (known) history of acute pressure rise also showed some form of VEP abnormality. The possible pathophysiological mechanisms operating in both affected and fellow eyes are discussed. It is concluded that, despite the presence of possible artefactual influences, the results probably reflect the presence of primary angle closure glaucoma.