Pyoderma gangrenosum is a neutrophilic dermatosis that may be associated with myeloid malignancies. Less information is available about the association of pyoderma gangrenosum with lymphoid malignancies. We present, to our knowledge, the first case of pyoderma gangrenosum associated with biphenotypic acute leukemia wherein the malignant cells show a phenotype specific for myelogenic and lymphocytic leukemia. Histopathologic examination revealed rather nonspecific features without involvement of leukemic cells in the skin lesions. Treatment with systemic steroids was followed by characteristically rapid healing of the skin lesion. (J Am Acad Dermatol 2001;44:530-1.)