The distribution and severity of the brain lesions were studied in 45 cases (age range: 26-84 years) of Wernicke's encephalopathy. The process was acute or subacute (active) in 24 cases and chronic (inactive) in 21. Cases with acute and subacute disease had more extensive and severe lesions than the chronic ones. The majority of the acute cases had lesions involving the mammillary bodies and thalamus and the subependymal structures along the third and fourth ventricles and the aqueduct. Only three acute cases had lesions restricted to the mammillary bodies. Among the chronic cases, the majority of the lesions were restricted to the mammillary bodies and the thalamus. Only two lesions extended as far down as the inferior collicles. Eleven out of 21 chronic cases has isolated lesions of the mammillary bodies. The affection of the mammillary bodies in chronic cases varied from barely visible to subtotal destruction of the tissue. Similarly, the lesions in the thalamus varied from slight gliosis in the dorsomedial nucleus to extensive nerve cell loss in several nuclei. There were similar variations in the severity of the clinical picture. Memory loss was recorded in three cases with isolated lesions of the mammillary bodies.