This long-term study analyzes the prognostic value of the quantitative urinary protein excretion during and following steroid administration, the renal functional status three years after the onset of disease, and the degree of histologic damage in adult patients with steroid-treated idiopathic nephrotic syndrome (INS). No patient who had a complete (proteinuria less than 0.1 gm/day) or partial (proteinuria less than 2.0 gm/day) remission during steroid administration progressed to renal failure. Furthermore, no patient in whom urinary protein excretion subsequently fell to below 2.0 gm/day ever progressed to renal failure. Only 3 of 49 patients in whom renal function was normal three years after the onset of INS developed renal failure. Finally, renal failure occurred in only 2 of 28 patients with mild abnormalities by light microscopy, compared with 12 of 21 patients with more advanced glomerular abnormalities. Thus, a partial, as well as a complete remission during steroid administration, subsequent reduction in proteinuria to below 2 gm/day, persistence of normal renal function beyond three years, or the presence of mild histologic abnormalities auger a favorable long-term prognosis in patients with INS.