Pyoderma gangrenosum is a relatively rare, destructive, inflammatory disease of unknown cause which may present as a purely cutaneous disorder or may be associated with an underlying internal disease (ulcerative colitis, Crohn's disease, rheumatoid arthritis, multiple myeloma, lymphoma and others). Four selected patients are described which reflect the clinical spectrum of this condition; these cases and a review of the literature serve as a background for analysis of pyoderma gangraenosum as an entity and a discussion of its pathogenesis.