Plasma low density lipoprotein (LDL) transport kinetics were determined from the disappearance of 125I-LDL injected into age- and weight-matched groups of 13 normal subjects, 20 mild diabetics, and 8 moderately severe diabetic patients (fasting plasma glucose less than 150 and greater than 150 mg/100 ml, respectively). In mild diabetics, LDL apo-lipoprotein-B (apo-B) synthetic rate (SR) was significantly greater than normal. The fractional catabolic rate (FCR), however, was also increased so that plasma LDL concentration remained normal. In moderately severe diabetics, LDL SR was normal but FCR was reduced resulting in increased plasma LDL cholesterol and apo-B concentrations. In normal subjects, moderate obesity was associated with increased LDL secretion. In diabetic subjects, however, changes in LDL turnover were of equal magnitude in obese and nonobese patients. In normolipemic and hyperlipemic mild diabetic subjects with equal degrees of glucose intolerance, both LDL apo-B SR and FCR were greater than normal. The magnitude of these increases, however, was lower in the hyperlipemic individuals. Stepwise regression analysis revealed that both LDL SR and FCR correlated positively and linearly with insulin response to glucose loading, but negatively and curvilinearly with fasting plasma glucose and glucose response. We propose that in noninsulin-dependent diabetes, mild hyperglycemia is accompanied by increased LDL turnover, despite normal plasma LDL levels, whereas moderately severe hyperglycemia is associated with decreased LDL catabolism, resulting in increased plasma LDL levels. These changes cannot be attributed to the presence of obesity or hypertriglyceridemia, and may relate to varying degrees of insulin resistance and decreased insulin secretion affecting plasma very low density lipoprotein (VLDL) secretion, VLDL conversion to LDL, and LDL catabolism. Both increased LDL turnover in mild diabetes and delayed removal of LDL in moderately severe diabetes could increase cholesterol ester availability to peripheral tissues, and may result in an increased risk of atherosclerosis.