Although the new World Health Organization (WHO) classification acknowledges "prefibrotic" phases, progression of myelofibrosis in chronic idiopathic myelofibrosis (cIMF) is controversial because there are only a few studies about sequential biopsy specimens, and they yield conflicting results. The conflicting results might be due to a mixture of different degrees of myelofibrosis and therapy regimens within the respective groups studied. To prove this hypothesis, we studied sequential bone marrow biopsy specimens from patients with cIMF and compared 3 groups with different degrees of myelofibrosis at initial diagnosis with a group of patients with primarily unfibrosed disease who met the WHO criteria for prefibrotic cIMF. Patients receiving chemotherapy were considered separately from patients without treatment. Our results favor a steady progression of myelofibrosis unrelated to therapy modalities, whereas confusing literature data can be explained: fibrosis may remain static or lessen, especially in more advanced stages of cIMF.