Med., 1987, 153 (2), 145 -150 We have shown previously that increased concentrations of plasma soluble fibrin monomer complexes (SFMC) and elevated fibronectin (Fn) levels are closely related to the development of diabetic microangiopathy. The purpose of the present study was to explore whether or not changes in plasma glucose levels could have an effect on these protein constituents. Plasma glucose levels of 25 uncontrolled diabetic patients were brought under control with insulin and serial measurements of SFMC and Fn were made over a period of 4 weeks. Glucose values fell from an average of 312 mg 100 ml to 160 mg 100 ml. Ten patients with macroproteinuria (i.e.>_ 0.5 g/24 hr) showed initially elevated plasma SFMC and Fn concentrations. These levels fell significantly over the 4 week observation period : from 13.6 mg/ 100 ml to 9.4 mg 100 ml for SFMC and from 38.4 mg 100 ml to 34.5 mg 100 ml for Fn. The remaining 15 patients had nearly normal levels of both SFMC (7.9 mg/ 100 ml) and Fn (31.1 mg 100 ml) and glycemic control brought no further reduction. The data indicated that a) elevated SFMC and Fn levels are indeed associated with diabetic microangiopathy, especially in the presence of macroproteinuria ; and b) adequate glycemic control is capable of normalizing the plasma concentration of these constituents.soluble fibrin monomer complexes ; fibronectin ; intravascular thrombin generation ; glycemic control It is well known that plasma soluble fibrin monomer complexes (SFMC) consist of the fibrin monomer, fibrinogen and fibrin degradation products. Fibrin monomer production essentially requires thrombin for the partial proteolysis of fibrinogen. In addition, the levels of SFMC closely correlates to that of /3-thromboglobulin (Iioka 1984). Hence, the increase in the levels of SFMC is an indicator of the intracapillary thrombin activation.Our recent studies revealed that plasma levels of SFMC and Fn in diabetics with advanced diabetic microangiopathy, especially in the presence of macroproteinuria, were higher than those seen in patients without diabetic microangiopathy.Moreover, plasma Fn level