Seizures complicate many neurosurgical diseases and, in many situations, their prevention is desirable. In an attempt to arrive at a rational approach to the purely prophylactic administration of anticonvulsants, existing data pertaining to four topics were examined: the danger posed by a single seizure, the incidence of seizures in a given disease state, the ability of anticonvulsant medication to prevent seizures, and the risks and benefits associated with pharmacological intervention. In general, where the risk of seizure exceeds 10 to 15% or where a single seizure may have disastrous consequences, anticonvulsant prophylaxis is recommended.