RATIONALE AND OBJECTIVES Signal changes of irradiated bone marrow are thought to represent fatty change. However, more recent studies have indicated that other factors may contribute to these signal changes. We performed animal studies to investigate magnetic resonance (MR) signal change factors in correlation with histologic findings. MATERIALS AND METHODS Forty-two male Wister rats were given single doses (3, 8, and 20 Gy) of irradiation. Magnetic resonance imaging (1.5 T) was performed using spin-echo technique. RESULTS In the acute phase (days 1–3), the T1 time was prolonged, probably secondary to decreased cellularity and edema. After this phase, the irradiated marrow showed marked T1 and T2 shortening that correlated with hemorrhage. In the chronic phase (after day 10), fatty replacement, fibrosis, and regeneration were observed. T2 times were stable in spite of regeneration because of the susceptibility effect of hemosiderin. CONCLUSION The MR signal changes of irradiated marrow reflect not only fatty replacement, but hemorrhage as well.