['Type I (insulin-dependent) diabetes mellitus is associated with an increased extracellular volume. Sodium restriction might seem a logical form of treatment but data on its renal effects is conflicting. We therefore studied the effects of sodium restriction on renal haemodynamics in uncomplicated Type I diabetes mellitus.', 'Uncomplicated Type I diabetic patients ( n = 24) and matched control subjects ( n = 24) were studied twice in random order: after a week of 50 mmol or after 200 mmol sodium intake, respectively. The diabetic patients were studied under normoglycaemic clamp conditions. Glomerular filtration rate and effective renal plasma flow were measured as the clearances of iothalamate and hippuran, respectively.', 'During liberal sodium intake, glomerular filtration, effective renal plasma flow and filtration fraction were similar between the diabetic patients and the control subjects. Sodium restriction decreased the effective renal plasma flow in both groups, whereas glomerular filtration rate only decreased in the control subjects. Consequently, in the diabetic patients, the filtration fraction was increased on low sodium (4.1 +/- 8.4 %, p < 0.05 vs liberal sodium). As a consequence, filtration fraction (24.0 +/- 2.6 vs 22.1 +/- 2.0 %, p < 0.05) and glomerular filtration (119 +/- 14 vs 110 +/- 13 ml/min, p < 0.05) were higher in the diabetic patients than in the control subjects during sodium restriction.'] Does short-term moderate sodium restriction induce relative hyperfiltration in normotensive normoalbuminuric Type I diabetes mellitus?