The purpose of this study was to develop a reliable and valid measurement for the identification of Internet addictive high school students. There were 615 subjects selected by a stratified sampling from the population of Taiwanese 10th to 12th graders. The final version of the Internet Addiction Scale for Taiwan High Schoolers (IAST) contained 20 items that were grouped into five factors: (1) Compulsive Use/Withdrawal; (2) Tolerance; (3) Related Problems: Resource Management; (4) Related Problems: Family/Health/Learning; and (5) Negation. The reliability of the scale as a whole and the subscales was satisfactory. Total scores of IAST demonstrated a normal distribution. Addicts classified by IAST showed a heavier usage in more Internet applications and perceived more negative influence of Internet toward their life than the addicts identified by a previous study and the non-addicts. The usage of the Internet negatively influenced life routines, health, parental relationship, and school achievement of the addicts. However, their teacher and peer relations were enhanced by the Internet use. The three most frequently-used Internet applications of the addicts were: the World Wide Web (5.79 hours/week), Bulletin Board Systems (3.85 hours/week), and chat rooms (3.61 hours/week). (Contains 11 references.) (Author/MES)