Overnight albumin excretion rates were measured in 940 diabetic patients, 416 with insulin dependent and 524 with non-insulin dependent diabetes, and in 106 healthy volunteers. A significantly higher number of non-insulin dependent diabetic patients had abnormal albumin excretion compared with the insulin-dependent group (X2 = 15.2, p less than 0.002). Ten per cent of non-insulin-dependent and 7 per cent of insulin-dependent diabetic patients had albumin excretion rates in the range 30-150 micrograms/min and thus were at risk of the cardiovascular and renal complications of diabetes. Six per cent of non-insulin-dependent and 5 per cent of insulin-dependent diabetic patients had albumin excretion rates above 150 micrograms/min and thus were entering the phase of clinical diabetic nephropathy. Multivariate analysis revealed that male sex and retinopathy in insulin-dependent diabetes, and systolic blood pressure and retinopathy and peripheral vascular disease in non-insulin-dependent diabetes, were significantly related to albumin excretion. Only one patient with insulin-dependent diabetes of less than 5 years known duration had an albumin excretion rate in the range 30-150 micrograms/min, whereas such an excretion rate indicating patients at risk was observed at all durations of non-insulin-dependent diabetes. It is possible that during the long silent phase of non-insulin-dependent diabetes, before diagnosis, significant renal damage occurred.