A series of recent observations have shown raised levels of plasma fibronectin (FN), an alpha 2-glycoprotein produced by vascular endothelia, in diabetic patients with retinopathy and overt nephropathy. However, there are no available data on urinary FN and behavior of its excretion in patients affected by diabetic nephropathy characterized by the presence of microalbuminuria. The main purpose of the present study was to investigate whether the urinary excretion of FN (U-FN) is associated with early diabetic nephropathy and other diabetic complications. Fifty-nine diabetic inpatients classified as type II and 15 age-matched control subjects were included in this study. The amount of U-FN, assessed as microgram/g creatinine/24 h, was significantly greater in the patients as a group (348.1 +/- 48.3), than in the controls (108.6 +/- 22.7, p < 0.01). Although patients with overt proteinuria showed an extremely high level of U-FN (1080.5 +/- 184.0; range 216.1 +/- 1726.8), mean U-FN tended to be higher in the group with microalbuminuria (262.4 +/- 21.9; range 101.9-591.9) than in the group without it (188.1 +/- 34.3; range 19.4 +/- 582.4, p < 0.08). In patients who did not have retinopathy and neuropathy, the U-FN was significantly higher in the group with microalbuminuria (222.5 +/- 28.5) than in the group without it (116.1 +/- 22.6, p < 0.01). A highly significant negative correlation existed between endogenous creatinine clearance values and the amounts of U-FN in the patients (r = -0.642, p < 0.01), while there was no such relationship in the controls (r = 0.167, p = NS).(ABSTRACT TRUNCATED AT 250 WORDS)