The objective of this study was to examine the associations of objectively measured and self-reported sedentary behavior during leisure time with academic performance and patterns of sedentary behavior with academic performance. This study was conducted with 1146 youth aged 12.5 ± 2.5 years in Spain during 2011–2012. Leisure-time sedentary behavior during out-of-school hours was assessed by accelerometry and self-report. Academic performance was assessed through school grades. ; Evaluación de la interacción entre genética y actividad física en el estado físico y la salud en escolares y adolescentes con Síndrome de Down: estudio de seguimiento a tres años Plan Nacional de I+D+i. Ministerio de Ciencia e Innovación (Ref. DEP2010-21662-C04-02). ; 2.893 JCR (2015) Q1, 24/151 Medicine, general & internal, 34/172 Public, environmental & occupational health ; UEM