Objective: Changes in thoracal kyphosis and lumbar lordosis, which are physiological sagittal plane curvatures of the spine, may cause many problems for adolescents in the future. This study aimed to compare the sagittal plane parameters of the spine of adolescents who are sedentary, athletes and diagnosed as adolescent idiopathic scoliosis (AIS), by evaluating them and to prevent possible injuries that may arise from changes in these curvatures in the presence of abnormal curvature. Materials and Methods: A total of 73 people (46 girls, 27 boys) who are 15-18 years old were included in the study. Of which, 27 were sedentary, 28 were athletes and 18 were individuals with adolescent idiopathic scoliosis (AIS). Gravity-dependent inclinometer (Baseline® Bubble® Inclinometer) was used to measure sagittal plane parameters. In addition, the Angle of Trunk Rotation (ATR) and Cobb angle values of the participants who were diagnosed as AIS were measured. Results: The kyphosis angle in the scoliosis group was significantly lower than both groups (p <0.05). The lumbar lordosis angle values of the scoliosis group were significantly higher than the athletes’ group (p = 0.020). The mean ATR value of the scoliosis group was 9.67° and the Cobb value was 21.11°. When analyzed by gender, the thoracal kyphosis angle was significantly higher in boys and the lumbar lordosis angle was significantly higher in girls (respectively p=0.000; p = 0.003). Conclusion: Our results showed that adolescents diagnosed with AIS have abnormal changes in physiological sagittal spinal parameters compared to sedentary and athletes; thoracal kyphosis angle was higher in boys and lumbar lordosis angle was higher in girls. Studies that also evaluated the spinopelvic complex, involving athletes with different sports branches and individuals with different curvature patterns are needed.