Fumaric acid preparations are used as longterm and effective treatment of psoriasis. Apart from gastrointestinal, dermatological and hematological side-effects, transient renal damage was observed during treatment with fumaric acid. The case of a 38 year old woman who was treated with fumaric acid (420 mg bid) for 5 years before she complained of fatigue and weakness. According to clinical laboratory she had developed severe proximal tubular damage. Hypophosphatemia, glycosuria and proteinuria persisted although medication was stopped immediately.