The dyslipidemia and insulin resistance of type 2 diabetes can be improved by aerobic exercise. The effect of 6 months supervised exercise on very low-density lipoprotein (VLDL) apolipoprotein B metabolism was investigated in patients with type 2 diabetes. Moderately obese patients (n = 18) were randomized into supervised (n = 9) and unsupervised (n = 9) exercise groups. All patients were given a training session and a personal exercise program and asked to exercise four times per week at 70% maximal oxygen uptake for 6 months. Patients in the supervised group had a weekly session with an exercise trainer. VLDL apolipoprotein (apo)B metabolism was measured with an infusion of 1-13C leucine before and after 6 months of the exercise program.Supervised exercise for 6 months resulted in a significant within-group decrease in percent hemoglobin A1c (P < 0.001), body fat (P < 0.004), nonesterified fatty acid (P < 0.04), and triglycerides (P < 0.05) and an increase in insulin sensitivity (P < 0.01). There was a decrease in VLDL apoB pool size (160.8 ± 42.6 to 84.9 ± 23.2 mg, P < 0.01) and VLDL apoB secretion rate (11.3 ± 2.6 to 5.5 ± 2.0 mg/kg·d, P < 0.05) with no change in fractional catabolic rate. In a between-group comparison, the decrease in VLDL apoB secretion rate in the supervised group did not achieve significance. This study demonstrates that in type 2 diabetes, a supervised exercise program reduces VLDL apoB pool size, which may be due to a decrease in VLDL apoB secretion rate.