The impact of improved glycaemic control on renal function in newly-presenting Type 2 (non-insulin-dependent) diabetic patients has not been adequately researched. Consequently, glomerular filtration rate and effective renal plasma flow and urinary albumin excretion rates were determined in 76 subjects (age (mean (SD)): 54 (9.5) years; 50 male) of an original cohort of 110 newly-presenting normotensive non-proteinuric Type 2 diabetic patients following 6 months treatment with diet alone (n = 42) or with oral hypoglycaemic agents (n = 34). Significant reductions were observed in (presentation vs 6 months): body mass index (p < 0.001); fasting plasma glucose (p < 0.001); glycated haemoglobin (HbA1) (p < 0.001); systolic blood pressure (p < 0.01); and diastolic blood pressure (p < 0.001). Glomerular filtration rate declined from 117 (22) to 112 (21) ml.min-1 (p < 0.01), with unchanged effective renal plasma flow (534 (123) vs 523 (113) ml.min-1) and filtration fraction (22.4 (3.0) vs 21.8 (3.4)%). Albumin excretion rate (median (range)) declined from 1.1 (0.1-34.7) to 0.5 (0.1-29.9) micrograms.min-1 (p < 0.01). Changes in glomerular filtration rate (delta values) were inversely correlated with presentation values (p < 0.001), and positive relationships were observed with delta effective renal plasma flow (p < 0.01), and delta glycated haemoglobin (p < 0.05). Type 2 diabetic patients with glomerular filtration rate values at presentation over 120 ml.min-1 demonstrated significant reduction in glomerular filtration rate (n = 31; p < 0.001), whilst those with original values less than 120 ml.min-1 remained unchanged (n = 45).(ABSTRACT TRUNCATED AT 250 WORDS)