Pyoderma gangrenosum is a poorly understood disease characterized by exacerbations and remissions of morphologically unique skin ulcers. It frequently is thought to be the cutaneous manifestation of an underlying systemic disease. In this review of pyoderma gangrenosum, the characteristics of the disease are described and a differential diagnosis is formulated. Associations with systemic diseases are made through a review of the literature. The pathophysiology of pyoderma gangrenosum and probable causes are considered, and special consideration is given to the immunologic mechanisms that may be operative in the disease. Finally the currently available therapeutic alternatives are reviewed.