ABSTRACT. Objective: The 21st birthday celebration is often associated with excessive alcohol consumption. The current study examined whether individuals consume more alcohol than anticipated during their celebration and whether situational factors contribute to prediction errors. Method: College students (N = 150; 50% female) who planned to drink during their 21st birthday celebration were contacted by telephone 1 week before their celebrations and asked about their birthday plans, including anticipated alcohol consumption. The week after the celebration, in-person semi-structured interviews and self-report measures were administered to obtain information about the 21st birthday celebration, including type and amount of alcohol consumed, pace of drinking, infl uential peer involvement, and engagement in 21st birthday traditions. Results: The majority of 21st birthday celebrants consumed more alcohol than they anticipated, with men showing greater prediction error than women. Situational factors were positively associated with the discrepancy between anticipated and actual alcohol consumption. Conclusions: Drinking shots, drinking at a fast pace, celebrating with infl uential peers, and engaging in 21st birthday traditions were associated with drinking more alcohol than anticipated during 21st birthday celebrations. Findings suggest future interventions that target situational factors could reduce excessive 21st birthday drinking. (J. Stud. Alcohol Drugs, 71, 180-183, 2010)