In 2018, the Intractable Hepato-Biliary Disease Study Group of Japan proposed that patients with cirrhosis and a Child-Pugh score of 5-9 be diagnosed as having ACLF if deterioration of liver function, defined by serum bilirubin level and prothrombin time, develops within 28 days after an acute insult. Disease severity would be classified into four grades depending on the extent of deterioration in organ function. This Study Group has since performed an annual nationwide survey of patients with ACLF who were diagnosed according to the proposed diagnostic criteria as well as those with conditions related to ACLF. Univariate and multivariate analyses revealed that the proposed diagnostic criteria were useful for identifying cirrhotic patients with an unfavorable outcome following an acute insult. Consequently, the Study Group determined that the proposed diagnostic criteria be formally used in both clinical practice and research.