No detectable abnormalities or changes in the post heparin plasma lipoprotein lipase (LPL) activity were found in fifteen “insulin independent” (stable) or sixteen “insulin dependent” (labile) diabetics, with or without hyperlipemia, in four patients with ketoacidosis and hyperlipemia both during the acute phase and after recovery, in two subjects with insulin induced hypoglycemia, and in two patients with gross hyperlipemia and diabetes during marked fluctuations in lipid levels in one, and during correction of the hyperlipemia in the other. Abnormalities in the LPL system, as reflected in the post heparin plasma LPL activity, appear to play a minor role, if any, in the pathogenesis of diabetic hyperlipemia in man.