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Introduction: The purpose of this study was to compare GH, insulin resistance index, lipid profile, cardiorespiratory function and their associations to leptin levels in inactive obese and lean young men. Materials and Methods: Thirty-eight obese and lean young men were studied. After 12 h fasting (at 8 A.M.), blood samples were collected to determine blood parameters levels and maximal oxygen uptake (as indicators of cardiorespiratory function) of subjects was also assessed. Results: Leptin and insulin levels, HOMA-IR (insulin resistance index) were higher, and GH and maximal oxygen uptake levels were lower, in obese versus lean men. Serum leptin concentrations were positively correlated to body mass, body fat percent, body mass index, insulin and HOMA-IR, and negatively correlated to GH levels and maximal oxygen uptake. No significant correlations were observed between serum leptin concentrations and systolic and diastolic blood pressure, fasting blood glucose, and lipid profiles in any of the groups. Independent t-tests were used to compare characteristics between obese and underweight groups, and relationships were calculated by Pearson’s correlation analysis, P values<0.01 being considered statistically significant. Conclusion: Obese and lean inactive young men had different levels of leptin, GH, insulin, insulin resistance index, cardiorespiratory function and body fat percent, of which body fat percent, insulin, and GH appear to the be more important determinant factors for leptin levels.