We have studied the long-term effects (9 months) in plasma lipoprotein concentrations during continuous subcutaneous insulin infusion (CSII) (n = 11, six females, five males) and compared these changes to conventional insulin therapy (CIT) (n = 12, six females, six males). The two groups were allocated to CSII or CIT randomly, and were comparable as regards lipoprotein values at the start of the study. There were initially normal total plasma cholesterol values in both groups (CSII group: mean plasma cholesterol 3.77 +/- 0.57 mmol/l, CIT group: mean plasma cholesterol 4.37 +/- 0.55 mmol/l, means +/- SD). Further, there were normal total plasma triglyceride values at the start of the study (CSII group: mean plasma triglyceride 0.86 +/- 0.23 mmol/l, CIT group: mean plasma triglyceride 0.84 +/- 0.26 mmol/l, means +/- SD). There were no alterations seen in total plasma cholesterol and total plasma triglyceride in either groups during a 9 months observation period. In the same period no changes in LDL and HDL levels were registered. The very low density lipoprotein (VLDL) was separated into VLDL-1 and VLDL-2 by its binding to heparin-sepharose columns. It was found that CSII treatment for 9 months resulted in a decline in VLDL-2-triglyceride values (0.18 +/- 0.07 mmol/l before versus 0.10 +/- 0.07 mmol/l after, p less than 0.05, means +/- SD) which was not seen in the CIT group. Decline in VLDL-2-triglyceride might delay the development of late diabetic manifestations.