A partial or complete replacement of the bone marrow stroma by fibrous tissue. It can be a primary bone marrow lesion as part of the chronic myeloproliferative disorders (chronic idiopathic myelofibrosis), a manifestation of acute myeloid leukemia (acute panmyelosis with myelofibrosis), or a secondary phenomenon due to bone marrow involvement by a metastatic tumor (e.g., metastatic breast carcinoma). --2003