OBJECTIVE: To determine the effect of exercise training (ET) on components of the insulin resistance syndrome (IRS) in obese children. DESIGN: Randomized, modi®ed cross-over study, with subjects assigned to one of two conditions: (1) 4 months of ET followed by 4 months of no-ET; or (2) 4 months of no-ET followed by 4 months of ET. Measurements were made at three time points: 0, 4 and 8 months. SUBJECTS: 79 obese, but otherwise healthy children (age: 7 ± 11 y, percent fat (%fat) 27 ± 61%). MEASUREMENTS: Plasma lipid and lipoprotein concentrations, plasma insulin and glucose concentrations; %fat; submaximal heart rate (HR) as an index of ®tness. EXERCISE TRAINING: ET was offered 5 daweek 40 minad. For the 73 children who completed 4 months of ET, the mean attendance was 80% (that is, 4 daweek) and the average HR during ET was 157 bpm. RESULTS: Signi®cant (P`0.05) group x time interactions were found for plasma triglyceride (TG) and insulin concentrations and %fat. The average change for both groups, from just before ET to just after the 4 month ET was À0.24 mmol Á l À1 for TG, À25.4 pmol Á l À1 for insulin and À1.6 units for %fat. When Group 1 ceased ET, over the following 4 month period the average change for insulin was 26.6 pmol Á l À1 and for %fat 1.3 units. CONCLUSION: Some components (plasma TG, insulin, %fat) of the IRS are improved as a result of 4 months of ET in obese children. However, the bene®ts of ET are lost when obese children become less active.