Pyoderma gangrenosum is strongly associated with inflammatory bowel disease and exhibits pathergy, occurring at sites ofprevious minor trauma. A patient is presented with a 21 year history of extensive ulcerative colitis, who developed pyoderma gangrenosum and arthralgia while receiving high dose corticosteroids for active ulcerative colitis. The arthralgia exhibited pathergy affecting particularly the left temporomandibular joint, which was stressed by an asymmetric bite, and the left elbow, which had been fractured many years previously. This prompted the hypothesis that neutrophils in this condition may be marginated, as a result of increased stickiness ofeither the neutrophil or the vascular endothelium. The introduction of heparin therapy was associated with rapid resolution ofthe arthralgia, pyoderma gangrenosum, and ulcerative colitis. (Gut 1995; 37: 585-588)