Is compulsive Internet use (CIU) an antecedent to poor mental health, a consequence, or both? Study 1 used a longitudinal design to track the development of CIU and mental health in Grade 8 (N = 1,030 males, 1,038 females, M[subscript age] = 13.7), 9, 10, and 11. Study 2 extended Study 1 by examining the kinds of Internet behaviors most strongly associated with CIU within males and females. Structural equation modeling revealed that CIU predicted the development of poor mental health, whereas poor mental health did not predict CIU development. Latent growth analyses showed that both females and males increased in CIU and mental health problems across the high school years. Females had higher CIU and worse mental health than males, and tended to engage in more social forms of Internet use. We discuss future directions for CIU intervention research.