In this research we wanted to examine the gender differences in self-image, self-esteem and some unwholesome behaviours (alcohol consumption, cigarette and marihuana smoking, overeating at a meal, televisin watching and computer use in spare time) in Slovenian adolescents. We used Offer Self-image questionnaire for adolescents, Rosenberg Self-esteem scale and unwholesome behaviour questionnaire that was made for the purpose of this research. In the research participated 392 high school students, aged around 15 and 18 years, approximately evenly represented by gender and age. Data was attained in class groups. The results show that boys have higher self-esteem and more positive self-image on all three areas of psychological self (impulse control, emotional tone and body self-image), on sexual attitudes and on psychopathology (the coping self). Girls have better self-image on two areas of social self (morals and vocational-educational goals). The differences in social relationships, the family self, mastery of the external world and superior adjustment were not statistically significant. On four of six unwholesome behaviours boys act unhealthier than girls (alcohol consumption, marihuana smoking, television watching and computer use in spare time). Gender differences in cigarette smoking and overeating were not statistically significant.