OBJECTIVE: The authors investigated the prevalence of Internet addiction (IA) in undergraduate medical students to identify possible associations with sociodemographics and Internet habits.METHODS: All students at the Aristotle University of Thessaloniki School of Medicine, Greece, were invited to complete the online Internet Addiction Test (IAT) along with sociodemographics and preferences on Internet activities.RESULTS: The authors received 585 responses after three reminders (23.5 % response rate). Mild IA was found in 24.5 %, moderate in 5.4 %, and severe in 0.2 %. In multivariable analysis, the odds to develop IA were increased with visits in Internet cafes (Odds Ratio [OR] 3.49, 95 % Confidence Interval [CI]: 1.45, 8.46), the use of Facebook (OR 2.43, 95 % CI: 1.35, 4.38), Twitter (OR 2.45, 95 % CI: 1.37, 4.39), and online games (OR 1.95, 95 % CI: 1.29, 2.94). Using e-mails seemed to be protective against IA (OR 0.59, 95 % CI: 0.37, 0.94).CONCLUSION: This is the first IA prevalence study in a European medical school. Early-detection systems and other ways to help students with pathological behaviors should be developed.