This study investigated the Turkish undergraduate university students' problematic Internet use (PIU) levels on different dimensions based on demographics (e.g., gender, Internet use by time of day), and Internet activities (e.g., chat, entertainment, social networking, information searching, etc.). Moreover, the study explored some predictors of PIU on different dimensions among the Turkish undergraduate students (N=610). The results indicated that the female students (N=358) had significantly lower PIU levels on all dimensions (i.e., social comfort, loneliness/depression, diminished impulse control and distraction) than those of the males (N=252). Furthermore, the results revealed that the students who use the Internet for entertainment (e.g., game) chat and social networking purposes had significantly higher PIU levels on all dimensions than the students who do not use the Internet for such purposes. However, the students who use the Internet for educational purposes had a significantly lower PIU level on distraction dimension, and the students who use the Internet for information searching demonstrated a significantly lower PIU level on the dimension of diminished impulse control. Moreover, multiple regression analyses revealed that Internet activities such as chat, entertainment and social networking were significant predictors of the all dimensions of PIU. (Contains 11 tables and 2 figures.)