A kidney and skin biopsy were performed on a patient who developed cryoglobulinemia, polyarthritis, a purpuric skin rash, and acute renal failure four years following jejunoileal bypass for morbid obesity. Morphologic studies revealed a diffuse glomerulonephritis characterized by the presence of numerous subendothelial deposits containing IgG, IgA, C3, Clq, C4, and properdin, and an acute dermal vasculitis associated with similar immune complex deposits. Identical immunoglobulin and complement components were present in the cryoglobulin. In addition, both the cryoglobulin and a renal biopsy eluate containing anti-IgG antibody and antibody against Klebsiella pneumoniae which were present in the patient's stool in large numbers. Combined therapy with steroids and chloramphenicol resulted in marked improvement in the patient's arthritis, skin rash, and renal function. The findings indicate that glomerulonephritis and dermal vasculitis due to the deposition of bacterial antigen-antibacterial antibody complexes may occur as part of a systemic immune complex disease complicating small intestinal bypass.