For a study, researchers sought to determine the link between the simplified magnetic resonance index of activity (MARIAs) and markers of inflammation by ileocolonoscopy (IC) in newly diagnosed transmural inflammation Crohn’s disease (CD) children. Retrospective analysis of 140 CD patients aged 6–18 years with baseline IC and magnetic resonance enterography (MRE) within 5 weeks of diagnosis. MARIAs was computed for each intestinal segment (terminal ileum [TI], ascending colon, transverse colon, descending colon, sigmoid colon, and rectum) using the formula (1×thickness > 3 mm) + (1×edema) + (1 fat×stranding) + (2×ulcers). Comparing MARIAs to IC results, sensitivity and specificity were obtained using receiver operating characteristic (ROC) curves. Using IC as the reference standard, the cutoff MARIAs of more than or equal to 1 identified TI segments with active inflammation with 84% responsiveness, 73% specificity, 85% positive predictive value (PPV), and 70% negative predictive value (NPV), as well as an area under the curve (AUC) of 0.782 (95% CI of 0.689–0.802). MARIAs of more than or equal to 2 identified TI segments with severe lesions with a sensitivity of 87%, a specificity of 76%, a PPV of 87%, an NPV of 76%, and an AUC of 0.814 (95% CI: 0.712–0.905). All segments of the colon exhibited inadequate sensitivity. In children with freshly diagnosed CD, the Marias were practical and accurate in reflecting disease activity in the TI but not in the colon. Although MARIAs might be effective for tracking TI illness activity over time, IC and MRE were still required for a comprehensive evaluation.