Lack of daily physical activity is a contributing factor to the adolescent obesity epidemic. Few studies have been conducted to examine the relationship between accelerometer-determined physical activity and body composition during the summer months in adolescents. Purpose: The purposes of this study were: 1) to determine the relationship between physical activity and body composition in adolescents, and 2) to determine if physical activity differs between adolescents with normal body mass index (BMI) compared to at risk/overweight adolescents. Methods: Thirty-five adolescents [Boys: n = 19, age = 14.00 ± 1.33 years, BMI = 21.83 ± 3.60 kg/m2, percent body fat (%BF) = 22.10 ± 9.00; Girls: n = 16, age = 14.25 ± 1.61 years, BMI = 19.19 ± 3.00 kg/m2, %BF = 21.77 ± 6.40; 74% Caucasian] participated in this study during the summer months. Participants were given an accelerometer and instructed to wear it at their waists during all waking hours for seven consecutive days. In addition, height and weight were assessed to determine BMI, and %BF was measured using air-displacement plethysmography. Participants were categorized into two groups based on BMI for age classification: 1) normal (BMI for age £ 5th to <85th percentile) and 2) at risk/overweight (BMI for age ³85th percentile). Descriptive statistics, correlations, and t-tests were computed. Results: Participants wore the accelerometers an average of 6.06 ± 0.77 days for 15.71 ± 2.37 hours/day. Overall, they averaged 335.94 ± 156.10 counts/minute and 312,015.42 ± 141,589.34 total counts/day. Counts/minute (p = .22), total counts/day (p = .11), and %BF (p = .91) did not differ between boys and girls; however, BMI differences were significant (p = .03). The correlations between physical activity and body composition variables ranged from -0.35 to 0.11 for boys, and from -0.21 to 0.14 for girls. Counts/minute (p = .90) and total counts/day (p = 0.69) did not differ between the normal and at risk/overweight groups. Conclusions: The results of this study indicate that ambulatory physical activity may not differ during the summer months between: 1) boys and girls, and 2) normal and at risk/overweight adolescents. Although the correlations between physical activity and body composition were low, the results suggest that the relationships may vary between boys and girls. Keyword(s): exercise/fitness/physical activity, health promotion, obesity issues