Renal function and biopsies were studied in 18 patients, 7 to 108 months after intestinal bypass. Enteropathy was found in 12 and hyperoxaluria in 16. Every biopsy showed a type of focal interstitial nephritis, tubular atrophy, fibrosis, and glomerular hyalinization. Damage ranged from minimal to extensive and renal function from normal to end-stage failure. Tubular injury had resulted partly from oxalate deposits. However, in 10 patients no oxalate crystals were seen. In eight others, most of the damaged areas were remote from crystal deposits. Immunoglobulin M and C3 deposits, found in glomerular capillaries and the mesangium in six of 11 specimens, and the presence of circulating immune complexes in five of 10 patients, in addition to the extraintestinal organ involvement, suggested immune complex mesangial injury as one factor in bypass nephropathy. With progressive impairment of renal function, a biopsy appears justified. If damage is significant, the bypass should be dismantled.