The respective rôles of arterial blood pressure and metabolic control in different stages of diabetic nephropathy were analyzed retrospectively in 52 sequentially-followed Type 1 (insulin-dependent) diabetic patients. A negative correlation was found between median post-prandial blood glucose and median duration of diabetes until onset of persistent proteinuria (p less than 0.01). Systolic blood pressure was higher in patients who subsequently developed persistent proteinuria than those who did not (140 versus 121 mmHg; p less than 0.05), but duration of the interval until onset of persistent proteinuria was not related to blood pressure. After onset of persistent proteinuria, hypertensive diabetic patients developed elevated serum creatinine concentrations more frequently than normotensive diabetic patients (67% versus 14%, p less than 0.05). In these patients, the delay until elevation of serum creatinine concentration was negatively correlated with blood glucose (p less than 0.01). Once serum creatinine was raised, decay of renal function occurred faster in patients with persistent than intermittent hypertension (p less than 0.05). No effect of metabolic control was demonstrable at this stage of nephropathy. It is concluded that metabolic control determines the early course of diabetic nephropathy, whereas blood pressure is more important in advanced stages of nephropathy.