We describe two patients who developed acute renal failure during therapy with fumaric acid-esters. Histologic findings after renal biopsy in one patient were compatible with the diagnosis of acute tubular necrosis (ATN), and renal function was restored after cessation of the medication. The histologic diagnosis in the other patient was tubulo-interstitial nephritis (TIN), possibly reactive to ATN. The recovery of renal function was incomplete after 9 months. Two other patients had deterioration of renal function and proteinuria during therapy with fumaric acid-esters. The symptoms were completely reversible in one patient after discontinuation of the medication, and incompletely reversible in the other. The literature is reviewed and a comparison is drawn with the maleic acid model in the rat.