BackgroundEmerging adulthood is a critical neurodevelopment period in which extreme drinking has a potentially pronounced neurotoxic effect. Therefore, extreme drinking, even a single episode, could be particularly harmful to the developing brain’s structure. Relatedly, heavy alcohol use in emerging adults has been associated with structural brain damage, especially in the corpus callosum. However, it is unclear whether and how much a single extreme drinking episode would affect brain morphometry.MethodsFor the first time in the literature, the current study prospectively examined the impact of an extreme drinking episode (i.e., twenty‐first birthday celebration) on the brain morphometry of emerging adults immediately following their birthday celebration (n = 50) and approximately 5 weeks post–birthday celebration (n = 29).ResultsWe found evidence that a single extreme drinking episode was associated with structural changes immediately post–birthday celebration. Specifically, higher twenty‐first birthday estimated blood‐alcohol concentration was associated with decreased volume of the posterior and central corpus callosum immediately post–birthday celebration. This extreme drinking episode was not associated with further structural changes, or recovery, 5 weeks post–twenty‐first birthday celebration.ConclusionsOverall, results suggest that a single episode of heavy drinking in emerging adulthood may be associated with immediate structural changes of the corpus callosum. Thus, emerging adulthood, which is characterized by high rates of extreme drinking, could be a critical period for targeted prevention and intervention.