A three-color direct immunofluorescent staining method will be used in the evaluation of CD34+ cells. In our preliminary experience we observed four Flolan treated patients with PPH who had splenomegaly, anemia and/or thrombocytopenia. All of these patients had severe marrow fibrosis. The purpose of this study is to determine whether the incidence of the bone marrow fibrosis in this patient population is primary or secondary, and whether a secondary effect is a fibrosis that would be expected to be transient and resolve after discontinuation of Flolan.