The aim of the research was to determine the differences between real body mass index and perceived body-size among adolescent girls from selected secondary schools. We chose the classes from the 4-year secondary school in Bratislava. Girls involved in research were aged between 15 and 18 years. They have been measured in height and weight during the physical education lessons. We calculated the body mass index (BMI). While having them measured, they subsequently received a questionnaire with silhouettes of female figures. They identified one of the body-size about which they felt that most resembling their own body. We found the differences between real BMI and perceived body-size. From the results of our research, we can say that from all of girls (92) only 22 were seen the illustrated female body approximately at the same range as their real BMI. Test of good match - chi square not confirmed 1 % neither 5 % level of statistics significance.