Title: Alcohol-related harm in emergency departments: linking to subsequent hospitalizations to quantify under-reporting of presentations. Aims: To quantify the proportion of emergency department (ED) presentations that could be identified as alcohol-related when linking to a patient's subsequent hospitalization, compared with using ED data alone, and to assess that comparison according to the change in alcohol harm rates over time and potential variations within subpopulations. Design: A retrospective study using linked hospital administrative data to identify ED patients who had subsequent alcohol-related hospitalizations. Setting: Western Australia. Participants: A total of 533 816 Western Australian young people (246 866 females and 286 950 males), aged 12–24 years. Measurements: Whether or not presentations of young people to ED could be identified as alcohol-related, and for those that were not, how many had a subsequent alcohol-related hospitalization. Rates and proportions of alcohol-related harm for both methods of ascertainment were estimated by sex and Aboriginality across different age groups. Findings: Alcohol-related hospitalizations that followed an initial presentation at ED allowed the identification of an additional 19 994 alcohol-related presentations (95% increase). Linking to additional hospitalization information also resulted in significant variation in alcohol-related harm trends. In particular, trends in alcohol-related ED presentations for 21–24-year-old males were stable to slightly increasing using only ED data, but decreased after linking with hospitalization data (P < 0.05). Similarly, trends among Aboriginal persons aged 21–24 shifted from increasing using only ED data to being stable in comparison to presentations using subsequent hospitalizations (P < 0.05). Conclusions: Among young people in Western Australia, twice as many emergency department presentations could be identified as being alcohol-related using diagnosis information from subsequent hospitalizations compared with emergency department data alone. When supplemented with hospitalization data, trends in alcohol-related harm presentations become significantly different within some subpopulations compared with using emergency department presentation data alone.