Pyoderma gangrenosum is a rare, recurrent, painful and ulcerative skin disorder of unknown etiology, but is commonly associated with a variety of underlying systemic diseases of ulcerative colitis, rheumatoid arthritis, monoclonal gammopathies, Crohns disease, chronic active hepatitis and some hematologic diseases such as leukemia, thrombocythemia, dysglobulinemia and myeloma. We describe a 50 year-old woman who had pyoderma gangrenosum associated with idiopathic myelofibrosis. The findings of a bone marrow biopsy revealed proliferation of fibroblasts and a dense reticulin network, which were consistent with myelofibrosis. Steroid therapy resulted in a good response temporarily but, she died within a year after diagnosis. To our knowledge, our present case associated with idiopathic myelofibrosis is the first reported case in the Korean literature. (Kor J Dematol 1997;35(4): 781-785)