We present a case of secondary myelofibrosis in which magnetic resonance imaging (MRI) contributed to the diagnosis. On both T1- and T2-weighted images, bone marrow showed hypointensity. However, since blood transfusion therapy was performed prior to MRI, it was necessary to differentiate fibrosis from siderotic marrow. On gradient echo (GRASS) images, bone marrow also showed hypointensity, but not as prominent as that seen in siderotic marrow. After intravenous administration of gadolinium-dimethylene triaminepentaacetic acid (Gd-DTPA), enhancement of the marrow space may be induced by a fibrous change of the marrow. MR findings were confirmed by histological examination of the biopsy specimen.