We present a case of granulomatous interstitial nephritis and renal failure after a jejunoileal bypass for obesity. Improvement of the renal function occurred after reversal of the intestinal bypass. The renal biopsy showed an interstitial nephritis, oxalate crystal deposition and several aggregates of multinucleated giant cells related to the crystal material (granulomatous reaction). By ultrastructural and histochemical studies we demonstrated mitochondrial alterations in the tubular epithelial cells, and we suggested the proximal tubule origin of the giant cells. The association of the oxalate crystals with damaged tubules and giant cells suggests that the oxalate crystals are responsible for these alterations. The possibility of an associated immunological process as the cause of the interstitial nephritis cannot be excluded.