Technical Abstract: Objective: To determine whether physical activity and fitness differ among normal-weight-for-height, multiethnic, prepubertal girls whose parents are lean, obese, or both. Normal-weight white, black, and Hispanic prepubertal girls (mean age+/-standard deviation: 8.5+/-.4 years) participated in this study. Girls were recruited according to parental leanness or obesity defined as follows: girls with 2 lean parents (LN group; n=30); girls with 2 obese parents (OB group; n=27); and girls with 1 lean and 1 obese parent (LNOB group; n=44). Each child wore a heart rate monitor for two 24-hour periods, underwent a treadmill exercise test, answered an activity questionnaire, and completed energy expenditure measurements by basal calorimetry and doubly labeled water. The amount of time spent above 125% and 150% of basal heart rate (BHR) was calculated during each 24-hour period (n=84). Fitness (n=97), habitual physical activity (n=101), and physical activity level (PAL=total energy expenditure/basal metabolic rate n=101) were also measured. Results: The times spent above 125% and 150% of BHR were similar among LN, LNOB, and OB groups. Black girls spent less time than did white girls on the weekend above 125% BHR and above 150% BHR. No significant familial or ethnic differences in peak oxygen consumption or habitual physical activity were observed. PALs were as follows: LN=1.6 +/-.21; LNOB=1.64+/-.27; OB=1.58+/-.20; white=1.63+/-.23; black=1.58+/-.24 and Hispanic=1.65+/-.25. PAL was related to time spent above 125% BHR and 150% BHR (r=.31,.39). Conclusions: Physical activity and fitness of normal- weight prepubertal girls predisposed to obesity did not differ from girls without a family history of obesity.