A total of 424 adolescents (236 girls and 188 boys, mean age 15.34 years), who were attending a public high school in a large western Canadian city, participated in the study. A range of ethnic groups was represented, and 75 percent of the participants reported living in a two-parent home. A questionnaire administered to the sample obtained information on participants' demographics, health behaviors, and health history. A self-report questionnaire asked participants about past-year and lifetime self-harm. Participants who reported self-harm were asked to describe specific self-harm behaviors, and descriptions were coded into conceptually distinguishing categories. A 29-item self-report questionnaire was developed to assess motivations and functions of self-harm. Forty-two percent of the adolescents reported self-harm ideation, with significantly more girls (53 percent) than boys (28 percent) reporting these thoughts. Sixty-four adolescents (16 boys and 48 girls), constituting 15 percent of the total sample, reported self-harming behaviors. Categories of self-harm by frequency were cutting-type behaviors, hitting or biting self, abusing pills, eating disordered behaviors, reckless behavior, and bonebreaking (falling/jumping). Those who reported self-harm behavior also indicated increased antisocial behavior, emotional distress, anger problems, health risk behaviors, and decreased self-esteem. Gender differences were found across behavior and motivations. Findings support the coping or affect regulation model of self-harm. 3 tables and 57 references