The blood glucose/urinary glucose relationship was studied in 23 patients with Type 1 (insulin-dependent) diabetes. Glucose was infused intravenously in order to increase blood glucose concentration slowly and gradually. The renal threshold was recorded at the slightest trace of glycosuria and varied by a factor of 2 (from 6.0 to 14.3 mmol/l). The rise in blood glucose required to change the urinary output (0-1.1 mmol glucose/20 min) varied by a factor of 7 (1.1-7.6 mmol/l). The maximal rate of tubular glucose reabsorption varied by a factor of 2 (0.93-1.98 mmol/min). The renal threshold was negatively correlated with the creatinine clearance (r = -0.052, p less than 0.05), but was not correlated with diabetic control, age or duration of diabetes. The maximal rate of glucose reabsorption was negatively correlated with age (r = 0.47, p less than 0.05) and duration of diabetes (r = -0.54, p less than 0.05). In conclusion, urinary glucose excretion is dependent on both renal threshold and the splay and the slope of the blood glucose/urinary glucose excretion curve. Thus, the degree of glycosuria is of value as an index of diabetic control only when the blood glucose/urinary glucose relationship is known. The inverse correlation between renal threshold and creatinine clearance limits the usefulness of measuring glycosuria in patients with nephropathy.