A brain image averaging technique was applied to three-dimensional magnetic resonance images to identify visually detectable brain volume abnormalities in chronically alcoholic men, compared with healthy control men. This technique, which was based on pixel-by-pixel statistical probability mapping, revealed a dramatic reduction in the area of the corpus callosum in older alcoholics (age 45 years or older), relative to age-matched controls. Subsequent analysis used anatomical landmarks to outline the borders of midsagittal sections of the corpus callosum in a larger group of alcoholics and controls, who spanned the adult age range from 23 to 71 years. This analysis revealed significant reduction, most prominent in the genu and body, of total callosal area in the alcoholic group relative to the control group; the results were the same whether raw area measures or head size plus age adjusted measure were analyzed. Significant thinning of the callosal body in alcoholics is usually attributed to the relatively rare, nutritional-deficient condition, Marchiafava-Bignami disease. However, callosal thinning was present in vivo in chronic alcoholics without clinical symptoms of severe liver disease, amnesia, or alcoholic dementia. These data suggest that chronic alcoholism can be characterized by a continuum of graded brain dysmorphology, rather than classical alcoholic-related subsyndromes, such as Marchiafava-Bignami disease.