Glomerular filtration rate (GFR) is abnormally high in some, but not all, insulin-dependent diabetic patients. The potential importance of this hyperfütration lies in its possible link with later severe diabetic kidney disease. Inadequate glycaemic control is closely related to hyperfiltration but the mechanisms of the association are obscure. GFR and prevailing plasma glucose concentration were examined in a group of insulin-dependent diabetics without clinical proteinuria and in a group of non-diabetics, and their relationships observed using linear and multiple regression analysis. A positive correlation (r = 0.30, p < 0.05) is found between mean plasma glucose concentration and GFR up to a mean plasma glucose level of approximately 13.5 mmol/l. Glycaemia in excess of this degree tends to be associated with a lower GFR. Multiple regression analysis confirmed the independence of plasma glucose as a determinant of GFR (p < 0.05) at concentrations below 13.5 mmol/l. GFR declined significantly with age, but independently of diabetes duration, in the diabetic group (r = -0.48, p < 0.001). GFR in the control group showed a statistically non-significant decline with age