To assess immunologic factors in the pathogenesis of idiopathic minimal-change nephrotic syndrome (INS), serum immunoglobulin concentrations were measured in 37 children with this syndrome and compared with those found in 36 with nephrotic syndrome secondary to chronic glomerulonephritis (CGN). Serum IgG and IgA levels were significantly reduced in nephrotic patients with either INS or CGN, IgG averaging 18.5 and 25.9 per cent of normal (P less than 0.001) and IgA 59.8 and 44.1 per cent of normal (P less than 0.01) respectively. Values increased after treatment of INS with prednisone, but mean values remained low. Serum IgM concentrations in INS averaged more than twice normal before, during, and after successful treatment with steroids. Patients with CGN did not have equivalent elevations of serum IgM. Thus, the primary defect in INS may be immunologic and could consist of deficiency in the T-cell function that mediates conversion of IgM synthesis to IgG synthesis.