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Objective: To compare the diagnostic accuracy of diffusion-weighted (DW)-MRI with b-values of 50 s/mm 2 and 800 s/mm 2 for the detection of bone marrow metastases in children and young adults with solid malignancies., Methods: In an institutional review board-approved prospective study, we performed 51 whole-body DW-MRI scans in 19 children and young adults (14 males, 5 females; age range: 1-25 years) with metastasized cancers before (n = 19 scans) and after (n = 32 scans) chemotherapy. Two readers determined the presence of focal bone marrow lesions in 10 anatomical areas. A third reader measured ADC and SNR of focal lesions and normal marrow. Simultaneously acquired 18 F-FDG-PET scans served as the standard of reference. Data of b = 50 s/mm 2 and 800 s/mm 2 images were compared with the Wilcoxon signed-rank test. Inter-reader agreement was evaluated with weighted kappa statistics., Results: The SNR of bone marrow metastases was significantly higher compared to normal bone marrow on b = 50 s/mm 2 (mean ± SD: 978.436 ± 1239.436 vs. 108.881 ± 109.813, p < 0.001) and b = 800 s/mm 2 DW-MRI (499.638 ± 612.721 vs. 86.280 ± 89.120; p < 0.001). On 30 out of 32 post-treatment DW-MRI scans, reconverted marrow demonstrated low signal with low ADC values (0.385 × 10 -3  ± 0.168 × 10 -3 mm 2 /s). The same number of metastases (556/588; 94.6%; p > 0.99) was detected on b = 50 s/mm 2 and 800 s/mm 2 images. However, both normal marrow and metastases exhibited low signals on ADC maps, limiting the ability to delineate metastases. The inter-reader agreement was substantial, with a weighted kappa of 0.783 and 0.778, respectively., Conclusion: Bone marrow metastases in children and young adults can be equally well detected on b = 50 s/mm 2 and 800 s/mm 2 images, but ADC values can be misleading., (© 2022. The Author(s), under exclusive licence to International Skeletal Society (ISS).)