The relationship between fasting plasma insulin and serum lipid and lipoprotein levels was studied in 1982-1983 in Kuopio, East Finland in 225 patients with non-insulin-dependent diabetes mellitus (119 men and 106 women) and 124 non-diabetic controls (65 men and 59 women). Compared to the non-diabetic controls, diabetic subjects showed significantly lower levels of high density lipoprotein (HDL) cholesterol and high density lipoprotein2 (HDL2) cholesterol and higher levels of total triglycerides and very low density lipoprotein (VLDL) triglycerides. Fasting plasma insulin correlated significantly with total triglycerides and VLDL triglycerides and negatively with HDL cholesterol and HDL2 cholesterol in both male and female diabetic subjects and non-diabetic control subjects. The correlation between fasting plasma insulin and HDL cholesterol remained statistically significant in non-insulin-dependent diabetic subjects and in female non-diabetic control subjects after adjustment for body mass index, alcohol intake, physical activity, smoking, and fasting plasma glucose. The correlation between fasting plasma insulin and total triglycerides remained significant after adjustment for these variables only in females. By multiple stepwise linear regression analysis, fasting plasma insulin had an independent association with HDL cholesterol in female non-diabetic control subjects and in male diabetic subjects and with triglycerides in female non-diabetic control subjects and in female diabetic subjects. The results show that hyperinsulinemia is related to low HDL cholesterol and HDL2 cholesterol and high total triglycerides and VLDL triglycerides in both non-insulin-dependent diabetic subjects and non-diabetic control subjects. This effect of hyperinsulinemia on lipid and lipoprotein patterns may be one explanation why high plasma insulin can promote accelerated atherosclerosis, particularly in patients with non-insulin-dependent diabetes mellitus.