Identifying factors related to physical activity levels in young people is important for a more efficient health promotion. The aims of this study were to assess physical activity levels in a national sample of urban Spanish adolescents, and to examine the association between significant others' physical activity (father, mother, brother, sister, and close friends) and that of the adolescents. The present study comprised 2260 adolescents (1157 boys, 1103 girls) aged 13.0-18.5 years participating in the AVENA Study. Both the adolescents' physical activity and that of their relatives and close friends was assessed by questionnaire. The odds of being active were higher in boys than girls (odds ratio = 2.79, 95% confidence interval = 2.34-3.33) and tended to decrease across age groups in both boys and girls. Father's and older brother's physical activity was associated with boys' physical activity, while that of any significant other was associated with girls' physical activity. When both parents reported being active, boys had nearly two times higher odds of being active and girls had nearly three times higher odds of being active. The physical activity levels of Spanish adolescents are in line with those previously reported. Physical activity levels in girls are strongly related to the physical activity of any significant other, whereas physical activity levels in boys are only related to their male relatives' physical activity.