Technical Abstract: To determine whether physical activity and fitness change in girls with and without a predisposition to obesity from 8 to 10 years of age. Normal-weight girls (n=91) were recruited at 8 years of age according to parental body mass index (BMI): LN = girls with 2 lean (BMI<25kg.m-2) parents, LNOB = girls with one obese and one lean parent, and OB = girls with 2 obese (BMI>28kg.m-2) parents. A longitudinal study was undertaken with annual assessments at 8, 9, and 10 years of age. The primary outcomes were fitness ([OV0312]O2peak) measured by treadmill testing, and physical activity measured by heart rate monitoring and by questionnaire. Sedentary behavior was assessed by questionnaire. [OV0312]O2peak (mL.kg-1.min-1) did not change over time; however, [OV0312]O2peak (L.min-1 and mL.kg-1.min-1), time on the treadmill, and treadmill stage were different across groups (P<0.02). Girls with LNOB parents had a lower absolute [OV0312]O2 than the LN girls by 2.5 mL.kg-1.min-1 (P<0.05). The OB group had a 3.9 mL.kg-1.min-1 lower [OV0312]O2 than the LN group (P<0.001). The girls of LN parents also exercised longer on the treadmill (P<0.05) than girls with OB parents. The percent of the day spent active on the weekday and weekend did not change over time or between groups. Time spent watching TV during the school year and summer was similar over the study period and between groups. Our data suggest that fitness and physical activity remain fairly constant in girls from 8 to 10 years of age, but girls of obese parents tend to be less fit.