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Background: This study compared CT-based and ¹⁸F-fluoro-2-deoxy-D-glucose PET/CT (FDG-PET/CT)–based NCCN International Prognostic Index (NCCN-IPI) risk stratification in newly diagnosed diffuse large B-cell lymphoma (DLBCL). Materials and Methods: This retrospective study included 57 patients with newly diagnosed DLBCL who had undergone both (oral and intravenous contrast-enhanced full-dose) diagnostic CT and FDG-PET/CT. Diagnostic CT only and FDG-PET/CT were evaluated separately, and corresponding NCCN-IPI scores for the 2 datasets (NCCN-IPI_CT and NCCN-IPI_PET/CT) were calculated. Percentages of agreement and weighted k statistic between NCCN-IPI_CT and NCCN-IPI_PET/CT scoring with regard to the formation of low-, low-intermediate–, high-intermediate–, and high-risk groups were calculated. Results: In 47 of 57 patients (82.5%; 95% CI, 70.4–90.4), diagnostic CT alone was in agreement with FDG-PET/CT with regard to the formation of low-, low-intermediate–, high-intermediate–, and high-risk NCCN-IPI groups, but not in the remaining 10 patients (17.5%; 95% CI, 9.6%–29.6%). All NCCN-IPI disagreements between diagnostic CT and FDG-PET/CT were from the detection of additional lesions by the latter, most of them being bone marrow lesions. Agreement between NCCN-IPI_CT and NCCN-IPI_PET/CT with regard to the formation of low-, low-intermediate–, high-intermediate–, and high-risk groups was considered good (k=0.771). Conclusions: Although agreement between NCCN-IPI_CT and NCCN-IPI_PET/CT risk stratification is generally good, FDG-PET/CT results in higher NCCN-IPI risk stratifications in a non-negligible proportion of patients. Future studies should investigate the prognostic implications of these imaging-based differences in NCCN-IPI scoring.