The aim of this article is to assess the differences in sedentary and physical activity behavior, physical fitness and quality of lifein school-age children according to their adiposity state (BMI-SD= Body Mass Index Standard deviation) and gender. 352 children participated in the study (11.99 ± 1.5 years). Children were assigned to a normal weight group (NW= Normal Weight) (n=175) or to an overweight/obese group (OW/OB= Overweight and Obese) (n=177). The percentage of OW/OB was significantly superior (p<0.05) in boys (55.4%) than in girls. Boys reported spending 2:19 h/week (sd= 2:33; p<0.01) watching TV and 3:21 h/week (4:28; p<0.001) more than girls practicing moderate-high intensity physical activity. In contrast, in weight groups differences appeared only at high intensity activities to which NW children devoted 52minuts/week (2:33; p<0.01) longer than OW/OB children. NW and OW/OB groups presented significant (p<0.05) differences in all the physical fitness tests, except for the medicine ball toss one. Children's involvement in at least three hours a week of physical activity were associated to a lower prevalence of overweight or obesity, and to higher physical fitness in children.