Six patients from three centers had MRI and pathologic evidence of hippocampal sclerosis but no detectable hippocampal atrophy. Loss of normal internal structure and T1- and T2-weighted signal abnormalities allowed the MRI diagnosis of unilateral hippocampal sclerosis when hippocampal volume measurements were normal and symmetric. Although accurate hippocampal volume measurements will determine the most severely affected side in most cases, volume measurements or atrophy alone will not always detect all MRI-visible pathology. Additional detailed MRI assessment is required before structural abnormality of the hippocampus is excluded on the basis of volume measurements.