Hippocampal formation atrophy (HFA) developed in an adult, who did not have epilepsy previously, after the occurrence of new-onset partial seizures from acute thrombosis of an ipsilateral parietal venous angioma. There was no evidence of hippocampal injury, and the patient had only one brief, generalized tonic-clonic seizure. Although HFA progressed rapidly over 5.5 months, the partial seizures did not become prolonged or secondarily generalized. Evidence from the patient indicates that partial seizure activity can cause rapid and progressive hippocampal atrophy.