The present research examined how various locations and companions were associated with hazardous drinking during 21st birthday celebrations. The sample included 912 college students (57% female) who completed an online survey to examine 21st birthday drinking. Locations included bars, friends' houses, restaurants, outdoor barbecues, homes, parents' homes, and Fraternity/Sorority houses. Companions included friends, family members, casual acquaintances, roommates, significant others, Fraternity/Sorority members, and none (alone). Participants consumed an average of 7.6 drinks and reached an average eBAC of .15 during their 21st birthday celebrations. Locations accounted for 20%/18% of the variance in number of drinks and eBAC, respectively, whereas companions accounted for 23%/20% of the variance. Drinking with romantic partners was associated with less drinking, whereas drinking with Fraternity/Sorority members was associated with more drinking. Stepwise regressions combining locations and companions suggested that, overall, celebrating in a bar setting and with Fraternity and Sorority members were the strongest variables associated with drinking. With the exception of a bar setting, companions were the most important contextual factors associated with 21st birthday drinking.