Two sisters, aged 25 and 29 years, with generalized psoriasis guttata since childhood, developed nausea, upper-abdominal pain, loss of appetite, palpitations and flushes in the course of local and oral administration of fumaric acid esters. Because of these side effects the treatment was discontinued after about two weeks, and the symptoms disappeared. But proteinuria and haematuria were subsequently noted, creatinine concentration rose to 2.2 and 2.5 mg/dl, respectively, while creatinine clearance fell to 44 and 27 ml/min, respectively. Examination of urinary sediments and analysis of urinary proteins gave results compatible with tubular-interstitial renal damage. The abnormal renal functions and urinary findings proved reversible within three weeks.