Retrospective studies of military personnel and survivors of community disasters suggest a link between traumatic exposure and substance use. This is the first study to investigate this association prospectively in a military population. A representative cohort of members of the UK Armed Forces was recruited into a longitudinal study, with 1382 people surveyed at baseline, and 941 followed up around three years later. Alcohol and cigarette use were assessed on both occasions, and combat exposures during this time were assessed at follow-up. Alcohol consumption and the prevalence of binge-drinking increased over the course of the study. The increase in alcohol consumption was greater in those subjects who had been deployed, in particular in those who thought they might be killed (p=.010), or who experienced hostility from civilians while on deployment (p=.010). The effects of these combat exposures were strongest in those most recently deployed. In contrast, cigarette smoking declined during the three years of the study.