Oxalate nephrosis resulted in progressive renal failure in 4 patients after jejunoileal bypass for morbid obesity. In general, increased levels of oxalates in the blood and urine of such patients result from enhanced absorption of exogenous oxalates. Urinary calculous formation is determined further by concomitant deficiency of inhibitor substances, whereas oxalate nephrosis probably occurs as a result of oxalate deposition in renal interstitium via the blood stream. Clinical manifestations of oxalate nephrosis include pain, infection, hematuria and renal failure. Routine postoperative renal function studies and early renal biopsy in suspicious cases are urged to establish early diagnosis. Continued deterioration of renal function, despite therapy with oxalate restruction and oxalate binding agents, indicates a reversal of the bypass to preserve unaffected renal substance.