A survey of 138 patients with systemic lupus erythematosus (SLE) (113 with nephritis) was carried out following the finding of a patient with membranous nephropathy and IgA deficiency who later developed SLE. Two hundred random hospital patients and 143 patients with idiopathic glomerulonephritis were also studied. One patient with minimal change nephrotic syndrome had IgA deficiency, but the other 342 patients had normal IgA concentrations. In the SLE group, in contrast, 4 patients had IgA deficiency and another patient prolonged IgA deficiency following phenytoin treatment. These and other published data suggest that the association of IgA deficiency is about 20 times more common in patients with SLE and nephritis than in the general population. This association may be a manifestation of defective T-cell regulation, a failure to eliminate immune complexes in the absence of IgA, or both.