Hippocampal atrophy is a marker of Alzheimer disease (AD). It remains unclear whether this holds true for younger patients as well. Hippocampal volume was measured on MRI scans of 103 clinically diagnosed AD patients and 73 controls (aged 51 to 85 years). Aging and AD were independently associated with smaller hippocampal volume. Both young and old AD patients have hippocampal atrophy abnormal for age. Age-dependent criteria for hippocampal atrophy, suggestive of AD, are needed.