To evaluate the protective effect of dietary phosphorus restriction in an immunologic model of experimental renal disease, we randomized 24 Sprague-Dawley rats with established nephrotoxic serum nephritis into two groups. Group A animals (N = 13) were fed a diet with a normal phosphorus content (0.5% phosphorus), and group B animals (N = 11) received an identical diet low in phosphorus (0.04% phosphorus). Over the ensuing 133 days, group A rats developed progressive renal failure and had a mean serum creatinine concentration of 3.0 +/- 0.5 mg/dl at the time of death or completion of the study. In contrast, group B animals maintained near normal renal function and had a final mean serum creatinine concentration of 0.93 + 0.2 mg/dl (P < 0.001). Survival was markedly improved in group B animals (P < 0.001). Histologic damage was diminished greatly in group B animals by both light and electron microscopy; immunofluorescence was positive in all animals. Group A animals had increased kidney calcium concentration (30 +/- 6 mmoles/kg) when compared to group B animals (18 +/- 1 mmoles/kg) and animals with normal kidneys (13 +/- 1 nmoles/kg, P< 0.001). Conclusion. Dietary restriction of phosphorus retards functional deterioration and reduces histologic damage in experimental immunologic renal disease. The mechanism for this protective effect has not been elucidated.