The long-term metabolic effects of weight reduction on serum lipids and plasma insulin were evaluated in 32 obese children with a mean (+/- SD) relative weight of 160.9 +/- 20.1% before and after active treatment for 1 y and after observation for another 4 y. The obese subjects had higher serum triglyceride and plasma insulin concentrations initially but lower high-density-lipoprotein (HDL)-cholesterol concentrations and a lower ratio of HDL cholesterol to total cholesterol (TC) than 29 normal-weight children. There was a decrease of 15.8% in relative weight over the first year with a parallel decrease in serum triglyceride and plasma insulin, whereas HDL cholesterol and the ratio of HDL cholesterol to TC increased (P < 0.001). These changes remained stable over the second year. At 5 y the obese subjects still had a reduced weight (12.8% lower than initially) and higher HDL cholesterol. These results indicate that weight reduction in obese children is associated with a change in serum lipids toward normal and reduced peripheral hyperinsulinemia.