Pyoderma gangrenosum is an uncommon, ulcerative cutaneous condition of uncertain etiology. It is associated with systemic diseases in at least 50% of patients who are affected. The diagnosis is made by excluding other causes of similar-appearing cutaneous ulcerations, including infection, malignancy, vasculitis, collagen vascular diseases, diabetes, and trauma. In a process termed pathergy, new ulcerations may occur after trauma or injury to the skin in patients who already have pyoderma gangrenosum.