Diabetes is often accompanied by hypertension, microalbuminuria and decline in kidney function until renal failure. Diabetic nephropathy is the leading cause of chronic kidney disease. In nephrotic syndrome, aberrant filtration of plasminogen from plasma to preurine with subsequent activation to plasmin may proteolytically activate the epithelial Na+ channel (ENaC). We hypothesized that plasmin is lost to preurine in diabetic nephropathy and that plasmin in urine may activate ENaC current. This cross-sectional study included patients with type 1 diabetes mellitus with diabetic nephropathy (n=19) and without nephropathy (control, n=20). Patients were matched on age, gender and diabetes duration. Plasma, spot urine samples, 24 h urine collections and 24 h blood pressure recordings were obtained. 24 h urinary protein excretion was 1.8±0.5 (nephropathy, n=17) and 0.07±0.03 g/day (control, n=18, p=0.0007). Urinary plasmin(ogen)/creatinine ratio as assessed by ELISA was significantly higher in nephropathy than control group (172±1.86×10-3, n=19, vs. 3.48×10-1 ±1.32×10-3 µg/g, n=18, p<0.0001) and correlated with urine albumin concentration (p<0.0001). Plasma plasmin(ogen) concentration did not differ between nephropathy and control (140±7, n=18, vs. 152±9 µg/ml, n=19). Western immunoblotting supported the presence of urinary plasmin(ogen) in nephropathy, but not in control group. Preliminary data indicated that urine from nephropathy patients (n=3) increased ENaC current more than control group (n=5) as evaluated by whole-cell patch clamp on murine collecting duct cells (67.8±12.8 % vs. 16.5±3.9 %). Mean 24 h systolic, but not diastolic, blood pressure was significantly higher in nephropathy (144/79 mmHg, 95 % confidence interval (CI): 136.0-151.9/74.9-83.2) than control group (132/75 mmHg, 95 % CI: 126.2-137.4/71.8-77.8, p=0.0117) despite being treated with 3.0±0.4 (nephropathy) vs. 1.4±0.3 (control) antihypertensive drugs including diuretics. There was a significant correlation between mean 24 h blood pressure and urine plasmin(ogen) concentration (r2=0.18, p=0.0102). Furthermore, nephropathy patients dipped significantly less in systolic (7.4±1.8 % vs. 13.3±2.0 %, p=0.04) and diastolic blood pressure (9.1±1.6 vs. 16.1±1.9 %, p=0.008) at night than did the control group. It is concluded that aberrant presence of plasmin activity in urine in diabetic nephropathy is capable of activating ENaC and may contribute to insufficient hypertension control.