Emerging evidence suggests that transforming growth factor-β (TGF-β) is an important mediator of diabetic nephropathy. We showed previously that short-term treatment with a neutralizing monoclonal anti-TGF-β antibody (αT) in streptozotocin-diabetic mice prevents early changes of renal hypertrophy and increased matrix mRNA. To establish that overactivity of the renal TGF-β system mediates the functional and structural changes of the more advanced stages of nephropathy, we tested whether chronic administration of αT prevents renal insufficiency and glomerulosclerosis in the db/db mouse, a model of type 2 diabetes that develops overt nephropathy. Diabetic db/db mice and nondiabetic db/m littermates were treated intraperitoneally with αT or control IgG, 300 μg three times per week for 8 wk. Treatment with αT, but not with IgG, significantly decreased the plasma TGF-β1 concentration without decreasing the plasma glucose concentration. The IgG-treated db/db mice developed albuminuria, renal insufficiency, and glomerular mesangial matrix expansion associated with increased renal mRNAs encoding α1(IV) collagen and fibronectin. On the other hand, treatment with αT completely prevented the increase in plasma creatinine concentration, the decrease in urinary creatinine clearance, and the expansion of mesangial matrix in db/db mice. The increase in renal matrix mRNAs was substantially attenuated, but the excretion of urinary albumin factored for creatinine clearance was not significantly affected by αT treatment. We conclude that chronic inhibition of the biologic actions of TGF-β with a neutralizing monoclonal antibody in db/db mice prevents the glomerulosclerosis and renal insufficiency resulting from type 2 diabetes.