Our objective was to define glomerular filtration rate (GFR) and renal plasma flow (RPF) in Black Americans with non-insulin-dependent diabetes mellitus (NIDDM). This was a cross-sectional study of 71 Black NIDDM patients with diagnosed diabetes duration from 1 mo to 21 yr. Hyperglycemia was regulated and stabilized before patients were entered into the study. GFR and RPF were determined by the clearance of [l25I]iothalamate and 131I-labeled hippuran, respectively, with a constant-infusion technique and four urine collection periods. Hyperfiltration, as defined by a GFR of >140 ml · min−1 · 1.73 m−2, was found in 7 of 20 patients (35%) with newly diagnosed (<2 yr duration) NIDDM. The percentage of patients with hyperfiltration decreased with increasing duration of diagnosed diabetes. Decreasing GFR and RPF occurred with increasing duration of diagnosed diabetes. In conclusion, renal hemodynamic changes in Black Americans with NIDDM are similar to those known to occur in White populations with IDDM.