Acute cholangitis is a critical biliary infection that can swiftly evolve into sepsis and organ failure. Certain patients with mild acute cholangitis might advance to a more severe status. Identifying predictive factors for such exacerbation is of paramount importance. This study aimed to investigate whether the endotoxin activity assay (EAA) could serve as a predictive biomarker for the progression of mild acute cholangitis.We conducted a retrospective observational study at Yokohama City University Hospital, enrolling 200 patients hospitalized with acute cholangitis between May 2011 and June 2015. Patients with initially mild acute cholangitis were stratified into two groups based on their severity on Day 1: the stable group (remaining mild) and the exacerbation group (progressing to moderate/severe cholangitis). Clinical parameters were analyzed to assess risk factors for exacerbation.Among 74 patients with mild acute cholangitis at admission, 33 (44.6%) progressed to moderate/severe cholangitis within 24 h. Multivariate logistic regression analysis identified chemotherapy within 28 days [odds ratio (OR)=3.440, 95% confidence interval (CI)=1.170-10.100, =0.025], serum albumin levels (OR=0.303, 95%CI=0.094-0.975, =0.045), and EAA ≥0.4 (OR=3.880, 95%CI=1.210-12.500, =0.023) as independent predictors of disease exacerbation. A predictive equation was developed using the logistic regression model: log (P/1-P)=3.285-1.265×Alb (mg/dl) + 1.291 × (Chemotherapy within 28 days) +1.343 × (EAA ≥0.4) (P: the probability of exacerbation).EAA was identified as the most significant factor for exacerbating mild acute cholangitis. The combination of EAA, albumin levels, and a history of chemotherapy within the past 28 days suggests the potential to predict the progression of mild acute cholangitis to a more severe form.