PURPOSE: To characterize the magnetic resonance (MR) imaging features of myelofibrosis and compare them with bone marrow biopsy findings. MATERIALS AND METHODS: The authors retrospectively reviewed sagittal T1-and T2 weighted and short tau inversion recovery (STIR) images of the thoracolumbar spine of six patients (five males and one female, mean age 46) with biopsy-proven myelofibrosis. Marrow signal intensity of the thoracolumbar spine was classified with respect to those of muscle and fat, based on the consensus of two radiologists after visual inspection. These MR features were compared with the degree of fibrosis and marrow cellularity, as determined by bone marrow biopsy. RESULTS: In all patients, marrow signal intensity of the thoracolumbar spine was reduced onT1 and T2 weighted images (invariably low on T1 weighted images, low (2/6) to intermediate (4/6) on T2 weighted images). On STIR images, marrow signal intensity was variable (high (3/6) or low (3/6)), and this correlated with degree of fibrosis, not with marrow cellularity. The signal intensity of marrow with mild to moderate fibrosis was high on STIR images, while that of marrow with marked fibrosis was low. CONCLUSION: MR imaging features of myelofibrosis were characterized as low on T1 weighted images and low to intermediate on T2 weighted images. In addition, the signal intensity of STIR imaging correlated with degree of fibrosis.