Pyoderma gangrenosum, traditionally associated with chronic ulcerative colitis, is recognized by the rapidity of development of a liquefying and necrotizing ulceration. Three patients are described, each of whom developed an atypical, superficial bullous form of pyoderma gangrenosum. Study eventuated in the discovery of leukemia in each patient. Biopsy specimens of the skin lesions failed to show the histologic findings of lymphoma. To a degree, the course of the skin lesions paralleled the course of the leukemia, but the ultimate prognosis was poor, with rapid progression of the leukemia and death of the patient.