In diabetes mellitus, thickening of basement membrane in capillaries and small vessels is a well-known finding and important in the progression of diabetic microangiopathy. We evaluated whether the plasma levels of type IV collagen and fibronectin, which are important factors of basement membrane, are related with the presence of diabetic microangiopathy. Plasma type IV collagen and fibronectin levels were measured in 40 healthy controls (Mean ± SD, age; 50.3 ± 5.5 yr) and 94 diabetic patients (age; 52.4 ± 13.5 yr) with and without microvascular complications. The mean plasma levels of type IV collagen (5.3 ± 2.9 ng/ml) and fibronectin (474.4 ± 119.4 ug/ml) in diabetic patients were significantly higher (p < 0.01) than in healthy controls (3.7 ± 1.3 ng/ml and 319 ± 50.9 ug/ml). The mean plasma level of type IV collagen in diabetic patients with complications (6.6 ± 3.7 ng/ml) was significantly higher (p < 0.01) than in those without complications (4.3 ± 1.7 ng/ml) and became higher in more complicated patients. Furthermore, the severity of retinopathy and several indicators of nephropathy such as serum BUN, creatinine and proteinuria were closely associated with plasma type IV collagen level and a significant correlation was found between plasma type IV collagen and creatinine clearance (r = -0.31, p < 0.001). There was no significant difference in plasma fibronectin concentrations, however, between the diabetic patients with complications and those without complications.(ABSTRACT TRUNCATED AT 250 WORDS)