Background: An increasing number of people travel abroad for their holidays each year. New sexual relations while abroad may result in the acquisition and introduction of novel strains of sexually transmitted infections (STIs). Methods: We conducted a prospective cohort study to assess the impact of alcohol and drug use, and foreign travel on casual travel sex in students from a British university during the summer break in 2006. Results: Two thirds of students traveled abroad. They were more likely to consume alcohol (RR 1.59, 95% CI 1.17-2.16) and use drugs (RR 1.31, 95% CI 0.88-1.94), particularly Cannabis, and to have new sexual partnerships during holidays. They were also more likely to report sexual relations after holidays (RR 1.29, 95% CI 1.09-1.53). New partnerships were associated with being single, traveling abroad, drinking large amounts of alcohol, having previously had large number of sexual partners. The adjusted relative risk of developing new sexual partnerships with foreign travel was 2.70 (95% CI 1.11-6.61). Testing for a STI after the summer break was associated with both foreign travel (aRR 2.80, 95% CI 1.16-6.74) and younger age. Conclusion: People who travel abroad are more likely to engage in risk taking behavior and to develop new sexual partnerships during their holidays. They are also more sexually active on their return to the UK, increasing the chance of introducing new and resistant strains of STIs in the UK. These individuals are, however, also more likely to be tested for STIs.