This study is based upon an analysis of the hematologic and pathologic material from seven patients with acute myelosclerosis, as well as a review of the literature of 49 cases reported under this designation, or one of its synonyms. Patients with this disease characteristically present with pancytopenia, minimal or absent anisocytosis and poikilocytosis, and a fibrotic bone marrow showing hyperplasia and immaturity of all three cell lines, with particular prominence of megakaryocytes and their precursors. In addition, clinical splenomegaly is almost always absent, and the disease has a rapidly fatal course. We consider only one-fourth of the cases reported in the literature to have the clinical and hematologic features consistent with the diagnosis of acute myelosclerosis; the remainder represent a variety of myeloproliferative disorders, including chronic myelosclerosis with an accelerated terminal phase, acute myeloblastic leukemia with bone marrow fibrosis, myeloproliferative diseases that cannot be subclassified, and cases in which the data are insufficient for analysis. Using strict clinical and hematological criteria, acute myelosclerosis can be separated from other myeloproliferative disorders as a distinct clinicopathologic entity.