[Acute renal failure--"a well meant present from a friend"].

Acute allergic interstitial nephritis (AIN) due to non-steroidal anti-inflammatory drugs (NSAID) is a well known but rare adverse drug event. Here, we describe the case of a 70 year old woman with recurrent episodes of acute renal failure. A first episode of (AIN) occurred after the intake of mefenaminic acid tablets. A second episode of AIN occurred two years later, this time after transdermal application of diclofenac. Our case illustrates cross-reactivity between NSAIDs and shows that transdermally applied medication can cause systemic adverse events as well. Patients do not mention ointments because they often do not realize that ointments contain active substances, and physicians forget to ask.