To what extent dietary salt intake is involved in the pathogenesis of progressive renal diseases has never been fully understood in humans. To this aim, we investigated the relationship between urinary sodium excretion (under a low salt & low protein diet) and urinary protein excretion/renal function in patients with three major renal diseases: chronic glomerulonephritis(GN), diabetic nephropathy(DN) and nephrosclerosis(NS). The results were as follows; 1) A significant positive correlation was found between urinary sodium excretion (equivalent to the daily salt intake) and daily urinary protein excretion in patients with a GN and DN. However, no relationship was found between the two parameters in patients with NS. 2) Reduction in salt intake led to a significant decrease in daily protein excretion, the effect of which was prominent in patients with GN and DN. 3) A significant positive correlation was found between urinary sodium excretion and estimated protein intake(EPI) in all three groups. 4) There was a significant positive correlation between EPI and urinary protein excretion in DN, but not in GN. 5) Reduction in salt and protein intake(calculated as an EPI) ameliorates the slope of reciprocal creatinine concentration(1/Cr) in patients with GN and DN. These results indicate that slat restriction is strongly associated with the preservation of renal function in patients with GN and DN, suggesting that this dietary strategy can be a useful measure for retarding the progressive nature of these diseases. Of note is that both salt and protein restriction was renoprotective only in patients with DN. Thus, patients with GN and DN must be followed-up on the basis of a salt-restricted diet throughout their clinical course.