Body mass index (BMI) is used in the clinical assessment of adiposity in children and adolescents. Population-based, race-specific and age-specific curves of BMI for children and adolescents exist, but there are noknown sexual maturation-based BMI curves. The aim of this study was to investigate the effects of pubertal development (assessed according to the Tanner breast stage) on BMI in adolescent girls in a cross-sectional study. The study group comprised 167 healthy girls, between the ages of nine and 16 years, attending school near a hospital in Gerede, Bolu. A significant positive correlation was found between the Tanner stage of breast development and BMI (r = 0.79, p < 0.001). Age also had a significant influence on BMI (r = 0.69, p < 0.001). After controlling the effects of age, BMI was highly correlated with weight (r = 0.82, p < 0.001) and the Tanner breast stage (r = 0.49, p < 0.001), but not with height. The correlation between BMI and the sexual stage was also found to increase with increasing age. But when breast development was taken as a control parameter, BMI was not statistically associated with age or height. As a result, there was a significant variation in BMI with the Tanner breast stage in addition to the well known change with increasing age in adolescent girls. Developmental differences occurring in the same age may require that BMI be evaluated only within the same sexual stages in adolescence. This study indicates that the curves of BMI need to take into account the sexual maturation stage of adolescents.