To assess the severity of acute pancreatitis (AP) using computed tomography (CT) severity index (CTSI) and modified CT severity index (MCTSI), to correlate with clinical outcome measures, and to assess concordance with severity grading, as per the revised Atlanta classification (RAC). In this prospective study approved by the Institutional Review Board (November 2014 to March 2016), sixty patients with AP (as per the RAC definition) underwent contrast-enhanced computed tomography (CECT) 5-11 days (median 6 days) after symptom onset. Two radiologists, blinded to clinical parameters, independently assessed CTSI and MCTSI (differences were resolved by consensus). Clinical outcome parameters included duration of stay in the hospital and intensive care unit (ICU), presence of persistent organ failure (OF), evidence of infection, need for intervention, and mortality. We included 60 cases [36 males, age range 19-65 (mean 37) years]. As per the RAC, 26 patients had mild AP, 12 moderately severe, and 22 severe AP. According to CTSI and MCTSI, mild, moderate, and severe cases were 27 (45%), 19 (31.7%), 14 (23.3%) and 24 (40%), 10 (16.7%), 26 (43.3%), respectively. MCTSI was concordant with the RAC grading in 54 (90.0%), CTSI was concordant in 47 (78.3%), and both were concordant in 43 (71.7%) cases. Area under the receiver-operating characteristic (ROC) curves (AUROC) was compared by the Hanley and McNeil method. Both CTSI and MCTSI were significantly associated with outcome parameters (P < 0.001), except duration of ICU stay. Sensitivity, specificity, positive predictive value (PPV), and accuracy of CTSI for detecting moderate/severe disease were 97.1%, 100%, 100%, and 98.3% respectively, and of MCTSI were 100%, 92.3%, 94.4%, and 96.7% respectively. Both CTSI and MCTSI showed significant correlation with clinical outcome parameters, and good concordance with RAC grading of severity. MCTSI showed a higher sensitivity but lower specificity than CTSI in differentiating mild from moderate/severe AP.