Eighty-one adult patients with the idiopathic nephrotic syndrome were treated with prednisone, 60 to 120 mg, on alternate days. Treatment was continued with diminishing drug doses for up to 10 years. Biopsy specimens were categorized as showing lipoid nephrosis 36 per cent, focal sclerosis 12 per cent, diffuse proliferative 22 per cent and membranous nephropathy 30 per cent. Patients with systemic causes of the nephrotic syndrome were excluded. Proteinuria decreased to normal or to less than or equal to 3 g with a greater than or equal to 50 per cent decrease from base line in 83 per cent of the patients with lipoid nephrosis, 30 per cent of the patients with focal sclerosis, 50 per cent of the patients with diffuse proliferative nephritis and 71 per cent of the patients with membranous nephropathy. Improvement occurred in those with focal sclerosis, diffuse proliferative nephritis and membranous nephropathy only after prolonged treatment (14 to 15 months). Stable or improved renal function occurred in 97 per cent of those with lipoid nephrosis, 50 per cent of those with focal sclerosis, 73 per cent of those with diffuse proliferative nephritis and in 83 per cent of those with membranous nephropathy. Death or dialysis occurred in 12 per cent of the patients, and complications coincident with treatment occurred once every 12 patient years. Compared to other series of patients with the idiopathic nephrotic syndrome, therapy of our patients with prolonged alternate day steroids resulted in (1) decreased protein excretion, (2) maintenance of good renal function and (3) decreased number of complications of therapy.