Objectives: The purpose of this study was to assess the influence of sex and body mass index (BMI) on the thoracic kyphosis and lumbar lordosis of adolescents and to assess the reliability and agreement of the flexicurve method for these measurements.

Methods: The study included 217 adolescents of both sexes, aged between 11 and 15 years, who were students from municipal schools in the city of São José dos Campos in São Paulo. The measurement of thoracic kyphosis and lumbar lordosis angles was performed using the flexicurve method. Descriptive analysis of the data, analysis of covariance for comparison between groups (by BMI and sex), assessment of reliability, and intrarater agreement were analyzed.

Results: There was a significant difference between the groups by BMI and sex only for lumbar lordosis. The obese group had greater lumbar angles for both sexes (female sex: 32.6° ± 7.8° [eutrophic]; 37.7° ± 7.3° [obese]; male sex: 25.3° ± 7.3° [eutrophic]; 32.2° ± 7.3° [obese]). In the comparison between the sexes, the greatest lumbar angles were found in the female sex (female sex: 32.6° ± 7.8°; male sex: 25.3° ± 7.3°) among the eutrophic. Excellent intrarater reliability was found for thoracic kyphosis (intraclass correlation coefficient, 0.86) and moderate for lumbar lordosis (intraclass correlation coefficient, 0.72).

Conclusion: Sex and BMI were associated with lumbar lordosis in adolescents and were greater in individuals with obesity and female individuals. The flexicurve method was reliable and accurate for the assessment of thoracic kyphosis and lumbar lordosis in adolescents.