Visual evoked potentials (VEPs) obtained by using counterphase gratings were studied in adult patients with hydrocephalus and papilledema and in patients with papilledema due to brain tumor but without hydrocephalus. Transient VEPs were delayed only if hydrocephalic ventricular enlargement was present, while normal VEPs were recorded from patients with papilledema but without hydrocephalus. In three cases studied before and after treatment a reduction of the latency of the transient VEP corresponded to a reduction of ventricular size in patients with hydrocephalus. The steady-state VEPs similarly paralleled the recovery following therapy, but the response to specific spatial frequencies, the time of recovery, and measurements of interocular differences revealed that the two types of EP (i.e., recorded at slow and faster temporal rates) may tap different systems. The study of VEPs is proposed as a method of evaluating shunt patency in hydrocephalic patients.