BACKGROUND Heavy drinking and drug use is increasingly recognised as a problem in relation to holidays at nightlife resorts. However, studies conducted on the subject to date have relied on retrospective accounts of drinking, and little is known about the course of drinking, consequences and experiences during such holidays. METHODS A prospective study was carried out in Sunny Beach, Bulgaria, of 112 Danish tourists aged 16 to 25 years, 68% of whom were male. Participants completed questionnaires within one day of arrival at the resort, and subsequently on days 3 and 5 of the holiday. RESULTS-The response rate was adequate, with 81% providing complete data on all three occasions, and 93% providing complete data on at least two of the three occasions. During the holiday, participants cut down on their drinking significantly. Men drank more than women. The group of people with whom individuals travelled had a significant influence on their drinking. We also analysed predictors of several adverse outcomes. Individuals who travelled for the purpose of socialising drank more, whereas those who travelled to participate in sports activities drank less. CONCLUSION The study demonstrates the efficaciousness of prospective surveys as a methodology for onsite monitoring of substance use and other behaviours of young people during holidays. The study showed that young people drink like the members of the group they are with, but experience the consequences of drinking independently.