Microalbuminuria, an early feature of diabetic nephropathy, indicates intrarenal endothelial damage. In type 2 diabetes, microalbuminuria is strongly related to insulin resistance. We therefore investigated whether rosiglitazone, an insulin-sensitizing drug that is known to improve endothelial dysfunction, was able to improve intrarenal endothelial dysfunction and microalbuminuria. Nineteen type 2 diabetic patients participated in this double-blind cross-over trial. Nine patients with newly diagnosed disease without microalbuminuria were randomized to a treatment with rosiglitazone or nateglinide, each for 12 weeks. Ten patients with microalbuminuria were randomized to rosiglitazone or placebo, each for 12 weeks in addition to their previous antidiabetic medication. After each treatment, glomerular filtration rate (GFR), renal plasma flow, and filtration fraction were measured before and after blockade of nitric oxide (NO) by intravenous administration of N-monomethyl-L-arginine-acetate (L-NMMA). Ten healthy subjects served as control subjects. Type 2 diabetic patients at baseline showed glomerular hyperfiltration compared with healthy control subjects. Rosiglitazone reduced elevated GFR and filtration fraction toward control primarily in patients with microalbuminuria (GFR: 133.4 +/- 9.8 vs. 119.6 +/- 8.7 ml/min; filtration fraction: 23.2 +/- 1.7 vs. 20.5 +/- 1.6% before and after rosiglitazone, respectively; control subjects: GFR 111.7 +/- 8.6 ml/min, filtration fraction 20.4 +/- 1.5%). Rosiglitazone improved intrarenal NO bioavailability in type 2 diabetes toward control as shown by infusion of L-NMMA. Rosiglitazone reduced albumin excretion in type 2 diabetes with microalbuminuria from 116.5 +/- 31 to 40.4 +/- 12 mg/day. Rosiglitazone ameliorated glomerular hyperfiltration in early type 2 diabetes, improved NO bioavailability, and lessened renal end-organ damage in type 2 diabetes with microalbuminuria.