This paper summarizes the effects of Internet use on interpersonal behaviours among university students in Hong Kong. 2427 university matriculants in Hong Kong (mean age = 18.9, response rate 92.1%) completed an anonymous, self-administered questionnaire at the pre-enrollment health check-up. In the week preceding the survey, 99.4% of students had used the Internet with an average of 2.8 h/day; 14.9% reported heavy Internet use (> 4 h/day). Males (OR = 2.48) and Hong Kong-born students were more likely to be heavy users (OR = 1.67) while students with a religious affiliation (OR = 0.70), with a parent with tertiary education (OR = 0.68), or those in a romantic relationship (OR = 0.72) were less likely to be heavy users. Heavy users were less likely to socialize face-to-face (OR = 0.75) or talking to someone in person (OR = 0.61) in order to relieve feelings of stress, but were more likely to engage in online chatting (OR = 1.73) and online interactive games (OR = 2.43). They were also more likely to skip school (OR = 2.21), to be yelled at by family members (OR = 2.05) and to meet strangers from the Internet in person (OR = 3.31). There is strong evidence of the adverse effects of heavy internet use on interpersonal behaviours among Hong Kong university students, warranting further studies.