In a prospective study with cross-over design 20 patients with insulin-dependent diabetes mellitus of more than 2 years duration were treated for 6 months with continuous subcutaneous insulin infusion (CSII) and multiple insulin injections (MII). Metabolic control, platelet aggregability, thromboxane B2 levels in serum and plasma as well as antithrombin III (ATIII) activity and von Willebrand factor antigen were evaluated. A good metabolic control was obtained by both intensified regimens. No difference could be demonstrated between either platelet function tests or serum level of von Willebrand factor antigen during treatment with CSII and MII. However, the plasma level of ATIII activity was significantly higher (P less than 0.01) during MII treatment as compared to CSII treatment. There was no correlation between ATIII activity and daily insulin requirement or serum fructosamine. In conclusion, long-term metabolic control with MII has a favourable effect on ATIII activity in plasma. This may be important for a delay in onset and progression of diabetic vascular complications.