Developmental disorders of the hippocampal formation (HF) have been described in epileptic syndromes associated with lissencephaly, but HF malformations can be found without widespread cortical changes. We report 19 patients with partial epilepsy and abnormal HF patterns on magnetic resonance imaging (MRI). The changes consisted of incomplete folding with abnormal medial location along the choroid fissure, globular shape and/or verticalization, and were observed in the following three contexts: (1) diffuse disorder of neuronal migration (n=1); (2) temporal lobe malformation (n=5), including heterotopia, abnormal gyration, and, in 2 cases, reduced HF volume; and (3) apparently isolated HF changes (n=13, bilateral in 3 cases). The clinical features were heterogeneous in terms of severity of epilepsy and, when the focus could be determined, in localization (temporal or extratemporal). In 4 patients with apparently isolated HF changes, MRI was suggestive of both abnormal development and hippocampal sclerosis. In these patients, presurgical investigation and postoperative results suggested multiple epileptogenic foci, involving the frontal lobes in 2 cases. One HF specimen was large enough for the observation of developmental abnormalities corresponding to the changes seen on MRI. HF changes in shape and/or position should be included among the structural abnormalities associated with partial epilepsies. They may represent the visible part of a more extensive or more distant disorder of brain development.