Objective <p>To investigate DWI signal and IVIM for assessing hematological toxicity (HT) in patients undergoing chemoradiotherapy for rectal cancer.</p> Methods <p>This study retrospectively collected IVIM data from rectal cancer patients before and after concurrent chemoradiotherapy. The hematological grading of toxicity followed to the standardized criteria established by the WHO. Grading 2 and above is classified as HT2+, while grading below 2 is classified as HT2-. Pelvic bone marrow DWI signals were categorized into high and iso-low signal, and the signal intensity ratios (SIR) between marrow and muscle were calculated. Analyses were conducted to compare the clinical data, DWI signal, and IVIM parameters of pelvic bone marrow between the HT2 + and HT2- groups. Then, modelling was performed based on these meaningful factors.</p> Results <p>42 cases were enrolled in this study, 21 in each of the HT2- and HT2 + groups. There were no statistically significant differences in the clinical data between HT2 + and HT2- groups (all <i>P</i> &gt; 0.05). Differences were observed in pre-treatment DWI signal, pre-treatment SIR _{marrow−muscle}, and pre-treatment f value between the two groups (all <i>P</i> &lt; 0.05). The AUC of the pre-treatment SIR _{marrow−muscle}, f value, and their combination were 0.717, 0.737 and 0.816 respectively for predicting HT2+. However, no statistically significant differences were found among these AUCs (all <i>P</i> &gt; 0.05).</p> Conclusion <p>Pre-treatment high DWI signals and lower f-values in pelvic bone may be associated with HT2 + following chemoradiotherapy for rectal cancer. These preliminary findings suggest that DWI parameters may offer valuable insights for predicting hematological toxicity.</p>