Cutaneous necrosis is an infrequent complication of coumarin therapy. Skin necrosis has usually been reported in patients with congenital protein C deficiency or, less commonly, protein S deficiency. However, this complication may also occur with acquired and transient protein C and/or S deficiency. In coumarin therapy there is a relatively hypercoagulable state at the start of treatment, and most lesions appear between the third and sixth days. We describe a 75-year-old man receiving coumarin therapy (acenocumarol) for 7 years who was given a nonsteroidal anti-inflammatory agent (diclofenac) for a pain in his knee. Two days later, his renal function deteriorated and skin necrosis became evident. Biopsy showed histological changes consistent with coumarin-induced necrosis. Protein C and S levels were normal. We concluded that in our patient acute renal insufficiency aggravated by diclofenac treatment probably associated with an inadvertent withdrawal could have been the precipitating factor for transient protein C deficiency.