Internet gambling is a potential object of addictive behavior and consequently an important concern for public health. Epidemiological analyses of Internet gambling are necessary to determine the extent of public health threat. This paper reports the results of the first prospective epidemiological study of actual Internet poker gambling behavior. Participants were 3445 Internet gambling service subscribers who enrolled during February 2005. Data include two years of recorded poker outcomes (i.e., chips bought and sold) for each poker session played. Among our sample, we identified two subgroups of poker players. Approximately 95% of the sample bought a median of €12 worth of chips at each of two poker sessions per week during a median duration from first to last bet of six months. A smaller subgroup (i.e., 5%) of most involved poker players bought a median of €89 worth of chip at each of 10 sessions per week during a median duration from first to last bet of 18 months. In addition to level differences, we report the differences in patterns of behavior observed between these two subgroups. The analyses presented in this paper suggest that the majority of Internet poker players moderated their behavior based on their wins and losses. A minority of most involved players did not show such moderation. These results have important implications for both gambling-and addiction-related research.