Objective: To compare an automated intensity-based measure of medial temporal atrophy in Alzheimer disease (AD) with existing volumetric and visually based methods. Design: Longitudinal study comparing a medial temporal atrophy measure with 2 criterion standards: (1) total hippocampal (HC) volume adjusted for total intracranial volume and (2) standard visual rating scale of medial temporal atrophy. Setting: Cognitive disorders specialist clinic. Participants: Forty-seven patients with AD and 26 ageand sex-matched controls. Intervention: Subjects were scanned using volumetric T1-weighted magnetic resonance imaging at baseline and 1 year later. Main Outcome Measure: Automated Medial Temporal Lobe Atrophy Scale (ATLAS) score, derived from dividing mean intensity of a standardized perihippocampal volume by that of a standardized pontine volume. Results: Patients with AD had significantly reduced ATLAS scores and HC volumes and increased visual rating scores at baseline and repeat scanning. Rates of HC atrophy and decline in the ATLAS score were significantly higher in patients with AD compared with controls. The ATLAS scores were significantly correlated with HC volumes and visual rating scores. With specificity set at 85%, the sensitivities of HC volume and visual rating scale score were similar (84% and 86%, respectively), whereas ATLAS score had a lower sensitivity (73%). At repeat scanning, all 3 measures had similar sensitivities (86%-87%). Rate of decline in the ATLAS score required a similar sample size to HC atrophy rate to provide statistical power to clinical trials, but being automated, it is less labor intensive. Conclusions: Like the visual rating scale, ATLAS is a simple medial temporal atrophy measure, which has the additional advantage of being able to track AD progression on serial imaging.