The aim of this study is to examine the relations between physical activity of differing intensity and duration with body energy-balance hormone leptin in 10-12-year-old boys (N = 94) who participated in moderate-to-vigorous physical activity at least four to five times per week. The boys reported their physical activity using a questionnaire. They had normal body mass index (BMI, kg/ m2), and were at Tanner Stage 2 of development. Boys were divided into three subgroups by leptin levels: normal serum leptin (M +/- .5 SD, n = 44, 1.2-3.9 ng/ml), low leptin (< or = M - .5 SD; n = 31, < 1.2 ng/ml), and high leptin (> or = M + .5 SD; n = 19, > 3.9 ng/ml). There were significant differences between subgroups in anthropometric parameters and serum leptin levels, but not in physical activity. A significant correlation was found between leptin and moderate physical activity of at least five times per week for at least 30 minutes each time in the high leptin group (r = .61). In conclusion, the correlations between physical activity and leptin are weak; only moderate physical activity was correlated with leptin levels in the high leptin