The aim of the present study concerning patients with long-term insulin-dependent diabetes mellitus was to determine whether the serum lipid and lipoprotein concentrations differ in subjects with and without residual insulin secretion. We also investigated whether factors such as sex, smoking, physical activity and microvascular lesions were associated with particular lipoprotein profiles. C-peptide excretion (greater than or equal to 0.2 nmol) in 24-hour urine samples was used as an indicator of residual insulin secretion. Twenty-two pairs of patients with and without residual insulin secretion matched for age at onset and disease duration were participating in the investigations of glycaemic control and microvascular lesions. The HbA1c was significantly lower in C-peptide excretors than in the non-excretors (6.9 +/- 0.3 vs. 7.9 +/- 0.3%, p less than 0.025). The lipids and lipoprotein fractions were all within normal limits. The HDL2/3 ratio was significantly higher in C-peptide excretors than in non-excretors (1.72 +/- 0.28 vs. 1.10 +/- 0.09, p less than 0.05). Multiple regression analysis showed that factors, such as physical activity, body mass index and glycaemic control could explain more of the variation in the different lipid and lipoprotein fractions than residual C-peptide excretion alone. The only fraction correlating with C-peptide excretion was HDL3 cholesterol. It is concluded that minute residual insulin secretion per se is of minor importance for the regulation of lipids and lipoproteins. Glucose control and residual insulin secretion together with environmental factors seem to be of great importance for the regulation of the lipid and lipoprotein levels in insulin-dependent diabetes mellitus.