Objective: To study the incidence, etiologic risk factors, and outcome of biliary atresia. Study design: Register study using Swedish national health databases. The study population consisted of 1,204,791 children, corresponding to 99% of the entire cohort of Swedish children born between 1987 and 1997, with an end point of follow-up at 2 years of age. Cases with biliary atresia with and without major heart malformations were identified with indicators from various national health databases. Results: Eighty-five cases with biliary atresia were identified, the incidence being 1 in 14,000. A major heart malformation was found in 13 (15%) cases. In a multivariate analysis, 4 independent risk factors were identified: high maternal age (odds ratio OR = 3.0), parity of at least 4 (OR = 2.2), prematurity (OR = 2.9), and low birth weight for gestational age (OR = 4.7). No significant differences were found in the distribution of birth months. The outcome did not differ between the two groups with and without major heart malformations, nor with respect to any of the risk factors. Conclusions: The Swedish incidence of biliary atresia is similar to that found in other European countries. The identified risk factors may suggest the existence of a maternal vulnerability and the importance of viral infections transmitted from mother to fetus/neonate. (J Pediatr 2002;141:217–22)