Subjects at high risk for alcoholism have a greater propensity for externalizing behaviors and brain volume reductions of possible neurodevelopmental origin. Morphometric deficits in the corpus callosum (CC), which might reflect this neurodevelopmental abnormality, have been reported in other externalizing disorders such as attention deficit hyperactivity disorder, but not in subjects at high risk for alcoholism. The objective of the current study was to evaluate the CC morphometry in subjects at high risk for alcoholism. Magnetic resonance images of the CC in high-risk subjects (n=20) were compared with those of low-risk subjects matched to the high-risk subjects for age, sex, and handedness (n=20). Mid-sagittal areas of the CC, genu, body, isthmus and splenium were measured based on Witelson's method with good inter- and intra-rater reliability. Externalizing behaviors were assessed using the Semi-Structured Assessment for Genetics of Alcoholism-II. Total CC, genu and isthmus areas were significantly smaller in high-risk than low-risk subjects after controlling for age and intracranial area. The total externalizing symptoms score had a significant negative correlation with genu and isthmus areas. Smaller CC areas and their negative association with externalizing behaviors may represent yet another marker of susceptibility to alcoholism in high-risk subjects.