Alagille syndrome (AGS) is a well defined genetic disorder characterised by five major features. An autosomal dominant mode of transmission with reduced penetrance has been suggested by the analysis of a limited number of families. However there has been no statistical analysis. We report here the first segregation analysis of AGS, using 33 families collected through 43 probands. Segregation analysis of these families allowed us to conclude that AGS is transmitted as a dominant disorder with 94% penetrance and 15% of cases are sporadic. The expressivity of the phenotype was variable and 26 persons (15 parents and 11 sibs) were identified as presenting minor forms of the disease. These results are valuable for genetic counselling.