Our study investigated two groups of adult patients with established diagnoses of primary myelofibrosis (21 patients) and myelodysplastic syndromes (MDS) (21 patients). The objective was to assess fetal hemoglobin (HbF) concentration and to investigate correlations with organomegaly and extramedullary hematopoiesis and with the level of anemia and blood transfusion requirement. In all patients, the diagnosis was confirmed by histopathological examination. Patients with myelofibrosis were investigated by ferrokinetics using 59Fe. The percentage of marrow sideroblasts was assessed in patients with refractory anemia with ringed sideroblasts. Increased values of HbF were found to occur both in patients with myelofibrosis and with MDS, although a higher incidence and higher concentrations were evident in patients with myelofibrosis. Statistically significant increases in HbF concentration were found when there was accompanying organomegaly, as compared to patients without this feature. The average HbF concentration in both groups of patients under study was twice as high in cases with as in those without marrow fibrosis. The difference was statistically significant. Increased HbF levels appear to correlate with extramedullary hematopoiesis. HbF concentration did not correlate with the level of anemia or with requirement for blood transfusion.