Hypersecretion of growth hormone (GH) is a characteristic feature of Type 1 diabetic patients. In healthy subjects growth hormone is able to induce an increase in endothelial cell proteins such as fibrinogen and von Willebrand factor. Plasma concentrations of such proteins, which are markers of cardiovascular risk, are elevated in diabetic patients with microalbuminuria, suggesting endothelial cell dysfunction. In a randomized prospective study we therefore evaluated the possible effects of 1 year's treatment with a somatostatin analogue, octreotide, on lipoproteins and on endothelial function in Type 1 diabetes mellitus. Seven patients were allocated to treatment with a continuous subcutaneous infusion of 400 micrograms octreotide per day. Seven patients served as a control group. During treatment a decrease in plasma LDL-cholesterol (2.62 (2.17-3.11) (median (range] vs 2.00 (1.89-2.96) mmol l-1, p less than 0.05) and serum apolipoprotein A-I (1.47 (1.25-1.60) vs 1.23 (1.13-1.90) g l-1, p less than 0.05) was observed in the treated group. Furthermore a probable reduction during treatment in plasma concentrations of von Willebrand factor (1.72 (0.84-3.04) vs 1.24 (0.94-1.82) U ml-1, p = 0.08) and fibrinogen (11.3 (7.3-25.3) vs 8.1 (7.5-11.8) mumol l-1, p = 0.06) was found, and after withdrawal of treatment an increase towards the initial levels was seen. The platelet count declined (326 (301-612) vs 217 (206-400) x 10(9) l-1, p less than 0.01) during octreotide treatment and remained depressed 2 months after withdrawal.