Among a population of 4,436 Bassa, Kpelle, and Mano people in the Gbawein and Wroughbarh Clan region of Grand Bassa County, Liberia, 123 cases of epilepsy, ''See-ee'' in the local Bassa language, were identified. In 47% of the cases etiologic factors could be documented, and in 38% of all epilepsy cases infections involving the central nervous system precipitated the onset of the seizures. Positive family histories were obtained in 53% of the 123 epilepsy cases. The sex ratio was close to 1:1. Most frequently the seizure disorders occurred in siblings born to healthy parents. None of the language groups living in the area of investigation was preferentially affected by epilepsy. Bassa, Kpelle, and Mano patients were encountered proportional to the tribal distribution in the general population of Grand Bassa County. Seizure disorders appeared to be introduced in the area approximately 30 years ago as computed from the years of onset and retrospective information from elders in the local community. The epidemiologic data indicate that an environmental factor possibly interacting with genes determining the susceptibility of the host is involved in the etiology of highly prevalent seizure disorders in this county.