PURPOSE: Long-term survival of biliary atresia patients after Kasai's portoenterostomy is being increasingly reported. Although extended survival has been achieved for many patients, factors influencing outcome have not been defined clearly. The authors investigated 5-year survival rates and prognostic factors for survival after Kasai's portoenterostomy using univariate and multivariable methods. METHODS: The authors analyzed 5-year survival rates and prognostic factors in 56 patients who underwent Kasai's portoenterostomy during the period from 1991 to 2005 by the Kaplan-Meier model and Cox proportional hazards model, respectively. Both clinical factors and histological features of the liver and periportal fibrous mass were analyzed as prognostic factors. RESULTS: Younger ages of less than 90 days at surgery, clearance of jaundice and absence of post-Kasai cholangitis in 6 months were predictive of a favorable outcome. A significant difference between the live and the dead was found with regard to an average of 7 or more bile ductules in periportal fibrous mass in 200 HPF (P=0.013). The external diameter of the longest bile ductule in the periportal fibrous mass is not correlated to the prognosis (P=0.49). Independent factors for good prognosis were jaundice-free in 6 months and over 7 bile ductules density in periportal fibrous mass. CONCLUSION: Favorable outcome after Kasai's portoenterostomy for biliary atresia is determined by bilirubin clearance in 6 months after Kasai operation and seven or more bile ductules in periportal fibrous mass. The 5-year post-Kasai survival rate is 77.2%.