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. Keywords 21st birthday; legal drinking age; alcohol use; college students Drinking among young adults, particularly those in college, is an important public health issue (e.g., Hingson, Heeren, Winter, & Wechsler, 2005). From an array of risky collegiate drinking practices, hazardous drinking associated with the 21st birthday has emerged as a major concern for college administrators, student health service professionals, and public officials (e.g., American College Health Association, 2002). Furthermore, media reports have highlighted the problem by describing practices such as "21 for 21" and the "power hour" and by reporting the tragic deaths due to these risky practices (e.g., Zernike, 2005). Nevertheless, despite increased awareness of the problem, the prevalence and severity of and risk factors for 21st birthday drinking are not well characterized.We have identified only three published studies on this topic (Neighbors, Oster-Aaland, Bergstrom, & Lewis, 2006;Neighbors, Spieker, Oster-Aaland, Lewis, & Bergstrom, 2005;Smith, Bogle, Talbott, Grant, & Castillo, 2006). These studies provided preliminary evidence that a majority of 21st birthday celebrants consume alcohol and that their drinking levels are alarmingly high; however, these findings are limited by ascertainment biases of unknown magnitude resulting from low participation rates, 1 and it is likely that the prevalence and severity of birthday drinking are underestimated in these studies. In the present study we extend what is known about alcohol consumption on the 21st birthday by using data from a large prospective study of college drinking. Our first goal was to estimate the prevalence and severity of 21st birthday drinking in a large sample with high participation rates. In addition, our study examined the existence of the "21 for 21" phenomenon reported by the media. Our second goal was to examine risk factors associated with the occurrence and intensity of 21st birthday drinking. Although a range of possible variables could be considered, we elected to focus on those variables most consistently associated with drinking in college students (e.g., Jackson, Sher, & Park, 2006), including demographics, high-risk...