To combat hazardous drinking during 21st birthdays, a health-focused birthday card was mailed to 2,380 college students who later completed an online instrument to assess the campaign's impact. Students reported drinking more during their 21st birthday celebrations than on a typical weekend night. Men consumed more drinks, reached higher estimated blood alcohol concentrations, and tended to experience more negative consequences during 21st birthday celebrations. Students who overestimated their peers' 21st birthday drinking consumed more drinks and experienced more negative consequences than students whose perceptions were accurate or underestimates. Students who received the card before their birthday tended to drink less and experience fewer negative consequences during their birthday compared to those who received the card late. (Contains 1 figure and 11 tables.)