Using a selective immuno-chemical method, the activities of postheparin plasma lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) were measured in 7 children with newly diagnosed IDDM, 39 on a conventional subcutaneous insulin regimen (CSC), and 11 children receiving continuous subcutaneous infusien of insulin (CSII). The newly diagnosed untreated patients frequently had hypertriglyceridemia and a decreased serum HDL-cholesterol level, while they showed a mild, but not significant increase of the serum total cholesterol level. The insulin-treated patients (both on CSC and on CSII) had serum lipid levels similar to those in controls. LPL activity was decreased in untreated patients, and insulin treatment resulted in an increase in the LPL activity with a concomitant normalization of serum triglyceride and HDL-cholesterol levels. In contrast to the patients on CSII who had normal LPL activity, patients on CSC had an increased LPL activity. This may have been due to peripheral hyperinsulinemia. HTGL activity did not show any increase during the time of improved diabetic control. In conclusion; (1) serum lipid levels were normal both in the patients on CSC and CSIL (2) LPL activity was normal on CSII, but was increased on CSC and decreased in untreated patients.