Ultrastructural observations on 12 liver biopsies from 10 patients with arteriohepatic dysplasia syndrome (Alagille's syndrome) are reported. The electron microscopic changes in the liver in this condition are different from those seen in other forms of chronic intra- and extrahepatic cholestasis. In particular, the bile canalicular and pericanalicular changes classically observed in cholestasis are infrequently seen. When compared with other forms of intrahepatic cholestasis including syndromes associated with paucity of intrahepatic bile ducts, the ultrastructural changes in Alagille's syndrome appear to be distinctive. Bile pigment retention is found in the cytoplasm especially in lysosomes and in vesicles of the outer convex face of the Golgi apparatus (cis-Golgi), but rarely in bile canaliculi or the immediate pericanalicular region. These results suggest a block in the Golgi apparatus or in the pericanalicular cytoplasm.