ABSTRACT Aims To investigate predictors of hangover during a week of heavy drinking in young adults. Design Observational prospective study. Methods A total of 112 young Danish tourists were interviewed on three occasions during their holiday. They completed the Acute Hangover Scale and answered questions about their alcohol consumption and rest duration. The incidence of hangover was analysed as the proportion of heavy drinkers (i.e. those reporting drinking more than 12 standard units of alcohol during the night before) scoring above the 90th percentile of light drinkers (i.e. those who had consumed fewer than seven standard units the night before). We estimated the course and predictors of hangover using random effects regression. Results The incidence of hangover was 68% after drinking more than 12 standard units in the whole sample. The severity of hangover increased significantly during a week of heavy drinking and there was a time × number of drinks interaction, indicating that the impact of alcohol consumed on hangover became more pronounced later in the week. Levels of drinking before the holiday did not predict hangover. Conclusions Hangovers after heavy drinking during holidays appear to be related both to amount drunk and time into the holiday.