Treadmill stress testing was carried out according to the Bruce protocol on 43 control (C) and 25 obese (O) children of the same age. Body fat was 11.9+-0.8 and 37.1+-0.7% in G and O, respectively. Fasting plasma insulin (IRI), triglyceride and low density cholesterol (LDL-ch) levels were higher, high density cholesterol (HDL-ch) was lower in O compared to C (p<0.05). Exercise duration (ED) (C:749+-18.6, 0:529+-22.2 sec) and physical working capacity (PWC-170/kg LBM) (C:3.5+-0.2, 0:2.2+-0.2W/kg LBM) were decreased in the O (p<0.001). The relationship of anthropometric and atherosclerotic risk parameters (IRI and lipoprotein-ch classes) with the above fitness factors was investigated using multiple regression analysis. The relationship of blood pressure, ECG and atherosclerotic risk factors was also studied. The variation of ED was mainly influenced by rel.bw.(R=0.77) and by IRI and LDL-ch (R=0.7). LBM (R=0.53) and IRI and LDL/HDL-ch (R=0.59) accounted for the variation of PWC-170/kg LBM. It is suggested that beside body composition IRI level and LDL/HDL-ch ratio are important factors related to physical fitness.