Objective: To investigate whether long-term improved glycemic control by intraperitoneal insulin infusion normalizes the compositional abnormalities of triglyceride (TG)-rich lipoproteins in insulin-dependent diabetes mellitus (IDDM). Research design and methods: Seven subjects were studied before and 12-14 mo after initiation of treatment with the programmable implantable medication system (PIMS). Plasma TG levels were measured, and the composition of three TG-rich lipoprotein subfractions (Svedberg flotation [Sf] > 400, 100-400, and 20-100) were analyzed before and every 1.5 h for 7.5 h after ingestion of corn oil. Results: PIMS significantly improved glycemic control, as measured by mean blood glucose (P < 0.02), and HbA1 (P < 0.001, paired t test) levels. Weight loss was also observed during PIMS treatment. Significant changes occurred in the composition of TG-rich lipoprotein subfractions during PIMS treatment in both the fasting (P < 0.002) and the postprandial (P < 0.0001) state. Most changes were in the direction of nondiabetic values. PIMS treatment reduced the total cholesterol enrichment in IDDM subjects in all three subfractions in the postprandial state and the very-low-density lipoprotein subfractions (Sf 100-400 and 20-100) in the fasting state. Multivariate analysis showed that the compositional changes were affected by improved glycemic control, as assessed by both mean blood glucose and HbA1, whereas the very-low-density lipoprotein compositional changes were affected by both the improved glycemic control and body weight. Conclusions: In IDDM subjects during PIMS treatment, there was normalization of most abnormalities in the composition of fasting and postprandial TG-rich lipoproteins, including enrichment in total cholesterol, which is considered atherogenic.