To assess whether plasma and urinary endothelin-1 (ET-1) values are related to the severity of diabetic nephropathy, we measured plasma and urinary ET-1-like immunoreactivity (ET-1-LI) in 14 healthy subjects, and in 50 normoalbuminuric (group 1), 13 albuminuric (group 2), and 10 renally insufficient (group 3) patients with Type 2 diabetes. Plasma ET-1-LI values were significantly increased in group 3, and correlated positively with serum creatinine levels (r = 0.579, p < 0.01). Urinary ET-1-LI excretion in group 3 (49.3±7.3 pmol day⁻¹) was significantly higher than that in healthy controls (27.0±1.1 pmol day⁻¹) and in group 1 (32.2 ± 2.2 pmol day⁻¹), while that of group 2 (38.8 ± 5.9 pmol day⁻¹) was also higher than in healthy controls. A significant positive correlation between urinary ET-1-LI and serum creatinine values was also found (r = 0.297, p<0.05). Trend analysis showed significant linear and quadratic trends in the elevation of plasma ET-1-LI and a significant linear trend in urinary ET-1-LI levels from healthy controls to groups 1, 2 and 3. Our results demonstrate that an increase in plasma and urine ET-1-LI correlates with the severity of diabetic nephropathy.