Bacopoulou F, et al. - This cross-sectional study was performed to investigate differences in irisin concentrations between lean adolescents with polycystic ovary syndrome (PCOS) and age- and body mass index (BMI)-matched controls and examine the associations of irisin with core features of the syndrome. An association of irisin was found with the adolescents’ metabolic and reproductive characteristics and the hyperandrogenic phenotype of the syndrome.