Despite public recognition of the hazards of 21st birthday drinking, there is little empirical information concerning its prevalence, severity, and risk factors. Data from a sample of 2,518 college students suggest that 21st birthday drinking poses an extreme danger: (a) 4 of every 5 participants (83%) reported drinking to celebrate, (b) birthday drinkers indicated high levels of consumption, (c) 12% of birthday drinkers (men and women) reported consuming 21 drinks, and (d) about half of birthday drinkers exceeded their prior maximum number of drinks. Current problematic alcohol involvement and its typical correlates strongly predicted both the occurrence and severity of 21st birthday drinking. It is imperative that investigators consider a variety of potential interventions to minimize the harm associated with this rite of passage. (Contains 1 figure, 1 table, and 8 footnotes.)