Acute biliary pancreatitis (ABP) with cholangitis requires endoscopic retrograde cholangiopancreatography (ERCP) within 24 h to resolve ductal obstruction. However, this recommendation is based on the timing of emergency room (ER) visits. We wanted to determine the optimal timing of ERCP for ABP based on the timing of symptom onset, not the timing of the ER visit. We retrospectively reviewed 162 patients with ABP with cholangitis who underwent urgent ERCP (within 24 h of ER admission). Area under the receiver operating characteristic (ROC) curve (AUC) was analyzed to determine differences in complication rates according to time from symptom onset. A difference in ERCP-related adverse events (AEs) was identified, and Youden’s J statistic was used to determine a cutoff time from symptom onset (18 h). We compared mortality and complications based on this cutoff. Based on time to symptom onset, significantly higher rates of aspiration pneumonia (odds ratio [OR] 4.00, 95% confidence interval [CI] 1.15–13.92, P = 0.021) and post-ERCP hypotension (OR 11.9, 95% CI 1.39–101.33, P = 0.005) were observed in the ≤ 18-h group than in the > 18-h group. The study found that patients who underwent ERCP within 18 h of symptom onset is associated with an increased risk of ERCP-related AEs.