OBJECTIVE: Accurate evaluation of inflammation severity in ulcerative colitis (UC) can guide treatment strategy selection. The potential value of the pericolic fat attenuation index (FAI) on CT as an indicator of disease severity remains unknown. This study aimed to assess the diagnostic accuracy of pericolic FAI in predicting UC severity. MATERIALS AND METHODS: This retrospective study enrolled 148 patients (mean age 48 years; 87 males). The fat attenuation on CT was measured in four different locations: the mesocolic vascular side (MS) and opposite side of MS (OMS) around the most severe bowel lesion, the retroperitoneal space (RS), and the subcutaneous area. The fat attenuation indices (FAI(MS), FAI(OMS), and FAI(RS)) were calculated as the fat attenuation measured in MS, OMS, and RS, respectively, minus that of the subcutaneous area, and were obtained in the non-enhanced, arterial, and delayed phases. Correlations between the FAI and UC Endoscopic Index of Severity (UCEIS) were assessed using Spearman's correlation. Predictors of severe UC (UCEIS >/=7) were selected by univariable analysis. The performance of FAI in predicting severe UC was evaluated using the area under the receiver operating characteristic curve (AUC). RESULTS: The FAI(MS) and FAI(OMS) scores were significantly higher than FAI(RS) in three phases (all P < 0.001). The FAI(MS) and FAI(OMS) scores moderately correlated with the UCEIS score (r = 0.474-0.649 among the three phases). Additionally, FAI(MS) and FAI(OMS) identified severe UC, with AUC varying from 0.77 to 0.85. CONCLUSION: Increased CT attenuation of pericolic adipose tissue could serve as a noninvasive marker for evaluating UC severity. FAI(MS) and FAI(OMS) of three phases showed similar prediction accuracies for severe UC identification.