PURPOSE: To evaluate the role of bone marrow (BM) magnetic resonance(MR) imaging for assessment of theseverity of aplastic anemia (AA). MATERIALS AND METHODS: Eighty patients with AA, ranging in age from 16 to 44years underwent MR imaging. Fifty four patients had clinically severe AA (SAA),while in 26 the condition wasmoderate(MAA). Sagittal T1-weighted images (T1WI) and short tau inversion recovery(STIR) images of lumbarvertebral BM were analysed. Bulk T1, T2 and rho values (msec) were also measured, with mixed sequences. Signalintensity (SI) on both T1WI and STIR was classified into four patterns according to the amount of fatty marrow :pattern I, homogeneous fatty marrow ; II, fatty marrow with focal cellular nodules ; III, mixed fatty and cellularmarrow ; IV, cellular marrow with focal fatty nodules. These SI patterns and bulk T1, T2 and rho values of thelumbar BM were compared with the clinical severity of AA. RESULTS: On both T1WI & STIR sequences, MR imaging oflumbar vertebral BM in patients with AA showed various SI patterns. Pattern I, II and III were much frequently seenin the SAA group (48 of 54 patients on T1WI and 43 of 54 on STIR) and pattern IV was common in the MAA group (16of 26 patients on T1WI and 18 of 26 on STIR). The SI patterns of AA seen on both T1WI and STIR sequences closelycorrelated with clinical severity (x2 test, p = 0.0001). Bulk T1 value was significantly different between SAA andMAA ( SAA : 382.82 msec +/- 113.91; MAA: 517.99 msec +/- 151.92 ; t test , p = 0.0001). CONCLUSION: The SI patternseen on MR imaging, and T1 relaxation time of lumbar spinal BM can be useful for assessing the severity of AA.