Pyoderma gangrenosum is a rare condition which is sometimes associated with ulcerative colitis and Crohn's disease. The pathological basis is not completely understood but it may be caused by vasculitis. The lesions are typically said to have purple overhanging edges and a necrotic base. The condition may also occur with rheumatoid arthritis, multiple myeloma and leukaemia. The treatment of choice is systemic steroids but it may also respond to azathioprine, minocycline or clofazimine.