The diagnosis of arteriohepatic dysplasia may be difficult, particularly in very young patients with no family history, and the pathogenesis of the disorder remains obscure. It has been reported that the typical ultrastructural changes of cholestasis are scant in this condition and proposed that there is a failure of the hepatocytes to secrete bile into the canaliculi. In an attempt to expand on these observations and to test the value of ultrastructure as a differential diagnostic test for this condition, we chose two parameters felt to be associated with cholestasis that could be reliably and relatively simply determined by ultrastructural morphometry: canalicular dilatation and loss of canalicular microvilli. The values for these parameters were compared in arteriohepatic dysplasia with those in other infantile cholestatic conditions and with those in noncholestatic controls. The results of these morphometric studies corroborated the previous observation that canalicular dilatation in arteriohepatic dysplasia is minimal. There was, however, only a marginal difference in canalicular area between patients with arteriohepatic dysplasia and those with extrahepatic biliary atresia. There was significant overlap of values for canalicular area among all groups. Surprisingly, the studies showed no real loss of canalicular microvilli in any of the cholestatic conditions. The findings suggest that, at least for these two parameters, caution should be exercised in using qualitative electron microscopic evaluation as a diagnostic test for arteriohepatic dysplasia.