We used magnetic resonance imaging to determine the volume of the mamillary bodies in 9 patients with chronic Wernicke's encephalopathy, 7 patients with presumed Alzheimer's disease, and 37 control patients. The mean mamillary body volume (± standard error) was 21.3 ± 5.8 mm3 in Wernicke patients, 40.1 ± 3.7 mm3 in Alzheimer patients, and 51.7 ± 2.5 mm3 in control patients. Seven of nine (78%) patients with chronic Wernicke's encephalopathy had smaller mamillary bodies than 36 of 37 control patients and 7 of 7 Alzheimer patients. The decrease in mamillary body volume was related neither to patient age nor to degree of ventricular enlargement, and most likely reflects the mamillary body atrophy that is grossly apparent at autopsy in up to 81% of Wernicke patients. This technique provides a means of identifying the most specific macroscopic lesion of chronic Wernicke's encephalopathy.