We report on an elderly woman suffering from an acutely bleeding duodenal ulcer. Apart from old age and a history of gastrointestinal ulcer there were no risk factors detectable. However, clinical examination revealed that the patient had put a diclofenac hydroxyethylpyrrolidine plaster, which had been prescribed for lower back pain, inadvertently on a large ulcus cruris. The diclofenac serum concentration was 80 mu/l corresponding to a therapeutic serum level. This case demonstrates that false application of new pharmaceutical formulation may lead to inadvertent side-effects.