Substantial evidence suggests a role for genetic factors in the development of diabetic nephropathy in both type 1 and type 2 diabetes. In support of this view, non-diabetic relatives of type 2 diabetic patients with nephropathy have been found to display abnormalities of urinary albumin excretion rate (AER) both when measured at rest and during physical exercise. The aim of the present study was to assess the albuminuric response to physical exercise in non-diabetic relatives of type 1 diabetic patients with nephropathy. AER was measured from urine collections performed (i) overnight, (ii) during an oral glucose tolerance test (OGTT), and (iii) during a submaximal bicycle ergometer test in 21 and 24 non-diabetic siblings of type 1 diabetic patients with (DN+; AER > 200 microg/min) and without diabetic nephropathy (DN-; AER < 20 microg/min). No difference was found in AER (median [range]) measured overnight (DN+ vs DN-: 3.8 [1.3-24.1] vs 3.5 [2.0-21.0] microg/min; P=NS), during the OGTT (DN+ vs DN-: 6.3 [3.2-26.0] vs 4.8 [1.9-15.7] microg/min; P = NS) or during the exercise test (DN+ vs DN-: 44.8 [7.0-535] vs 30.0 [3.4-1614] microg/min; P = NS). In conclusion, we found no evidence of an exaggerated albuminuric response to physical exercise in non-diabetic relatives of type 1 diabetic patients with nephropathy. This differs from previous findings in type 2 diabetes and may suggest differences in the mode of inheritance of albuminuria between type 1 and type 2 diabetes.