PURPOSE: To assess the patterns of myelodysplastic syndrome(MDS) and aplastic anemia(AA) on MRI of the spinal bone marrow and to find the differential points between the two groups. MATERIALS AND METHODS: Fourteen patients with MDS(n=7) and AA(n=7) were studied using magnetic resonance imaging. Sagittal images from the lower thoracic and lumbar vertebral marrow were evaluated on Tl-weighted and STIR images. Five distinct patterns of signal intensity of the Tl-weighted and STIR images were classified. T1 and T2 relaxation times and T1 marrow/fat signal intensity ratio were measured and analyzed (t-test). The cellularity of bone marrow was evaluated on histologic slides. RESULTS: MDS showed homogeneously low signal intensity on T1WI and high signal intensity on STIR image, indicating hypercellular marrow, whereas AA showed relative high signal intensity on T1WI and low signal intensity on STIR image, representing fatty marrow. T1 and T2 relaxation time(T1 for MDS=750.26msec ± 177.50, T1 for AA= 413.21 msec ± 167.39 (p<0.000), T2 for MDS=91.86 msec ± 14.16, T2 for AA=81.44msec ± 15.31 (p< 0.001) and T1 marrow/fat signal intensity ratio(0.22 ± 0.048 in MDS, 0.30 ± 0.083 in AA(p<0.000)) revealed statistically significant difference between the two groups. CONCLUSION: Although the marrow aspiration and needle biopsy are mandatory in hematologic disease for diagnosis, there are limited in assessing the change of total marrow mass. Therefore MRI of bone marrow might be useful in distinguishing MDS from AA because of its ability of representation of total marrow mass.