Recently, we encountered a 39-year-old woman with typical epileptic amnesic syndrome. The patient underwent right anterior temporal lobectomy, which removed the right hippocampus. The patient's resected hippocampus showed typical histological features of Ammon's horn sclerosis (AHS) with dysmorphic neurons. In addition, the prominent balloon cells, admixed with dysmorphic neurons, were noted in the hippocampus. To our knowledge, this is the first reported case showing AHS with balloon cells. The presence of balloon cells reinforces the hypothesis that AHS itself might be a maldevelopment disorder.