Diabetic nephropathy is a common complication in patients with either type I or type II diabetes. The pathogenesis of diabetic nephropathy is thought to involve both metabolic and vascular factors leading to chronic accumulation of glomerular mesangial matrix. In this context, both transforming growth factor-β (TGF-β) and endothelin may contribute to these processes. To determine if diabetic patients demonstrate increased renal production of TGF-β and endothelin, aortic, renal vein, and urinary levels of these factors were measured in 14 type II diabetic patients and 11 nondiabetic patients who were undergoing elective cardiac catheterization. Renal blood flow was measured in all patients to calculate net mass balance across the kidney. Diabetic patients demonstrated net renal production of immunoreactive TGF-β₁ (830 ± 429 ng/min [mean ± SE]), whereas nondiabetic patients demonstrated net renal extraction of circulating TGF-β₁ (−3479 ± 1010 ng/min, P < 0.001). Urinary levels of bioassayable TGF-β were also significantly increased in diabetic patients compared with nondiabetic patients (2.435 ± 0.385 vs. 0.569 ± 0.190 ng/mg creatinine, respectively; P < 0.001). Renal production of immunoreactive endothelin was not significantly increased in diabetic patients. In summary, type II diabetes is associated with enhanced net renal production of TGF-β₁, whereas nondiabetic patients exhibit net renal extraction of circulating TGF-β₁. Increased renal TGF-β production may be an important manifestation of diabetic kidney disease.