To study the initial period of fat deposition in human obesity, we measured glycerol turnover in 12 children of 135-253% ideal body weight, who had continuously gained weight since the onset of obesity 2-9 yr previously. Hyperinsulinemia developed in these children depending on obesity duration (r = 0.74, P less than 0.01). Whole-body glycerol production was twofold greater in the obese children (311 vs. 156 mumol.min-1, P less than 0.01) and correlated with body fat (r = 0.67, P less than 0.005). Normalization of glycerol flux to fat mass revealed that the rate of triglyceride hydrolysis was in fact lower in the adipose tissue of obese children (9.4 vs. 17.7 mumol.min-1/kg body fat) and correlated with plasma insulin (r = 0.64, P less than 0.005). Euglycemic insulin clamps showed that the response of glycerol production to a unit increment in plasma insulin concentration was increased in obese children, suggesting increased insulin sensitivity of adipose tissue. As a direct consequence (r = 0.67, P less than 0.025) of their elevated plasma glycerol concentration (65 +/- 4 vs. 37 +/- 2 microM, P less than 0.05) obese children had an increased glycerol utilization by the whole body, as well as per unit of lean body mass (9.1 +/- 1 vs. 6.5 +/- 0.9 mumoles.min-1.kg lean body mass-1, P less than 0.025).