To investigate the possible role of cytokines in the mediation of glomerular injury in the nephrotic syndrome, the levels of interleukin (IL)-1β, IL-2, interferon (IFN)-α, IFN-γ, and tumor necrosis factor-α (TNF-α) were measured in patients with primary nephrotic syndrome. These patients had minimal change nephropathy (MCN), focal and segmental glomerulosclerosis (FSGS), or membranous nephropathy (MN) on biopsy. Cytokine levels were assessed by immunoradiometric assays, and specimens consisted of plasma, urine, and the culture supernate of mitogen-stimulated peripheral blood mononuclear cells (PBMC). Only TNF-α was found to be significantly elevated, in the plasma and urine of patients with FSGS and MN, above that found in healthy control subjects and patients with MCN. The elevation of TNF-a could not be shown to correlate with the length or severity of the nephrotic syndrome or with loss of body mass. IL-1β, IL-2, IFN-α, and IFN-γ levels were not elevated. In culture, mitogen-stimulated PBMC from all three groups of nephrotic subjects released an excess of TNF-α compared with controls, a response not consistently observed for the other cytokines measured. The findings of this survey of cytokine levels in nephrotic patients support the possibility that TNF-α may play a pathogenic role in the induction or maintenance of glomerular barrier dysfunction in humans.