Angiomyolipoma occurs rarely in the liver, with only 25 previous cases being reported in the English literature. The article describes two additional cases, one of which was multicentric, with results of ultrastructural and immunocytochemical studies. Many of the tumor cells contained numerous electron-dense granules, some with transverse striations like those found in melanosomes. Both tumors stained positively for S-100 protein and melanoma-specific antibody HMB-45. One case also expressed vimentin and neuron-specific eno-lase. Both were negative for cytokeratin, carci-noembryonic antigen, a-fetoprotein, desmin, muscle-specific actin, factor VIII antigen, and chromogranin. Comparison of our ultrastructural findings with those of classic renal angiomyolipoma raises the possibility that the melanosomelike structures may represent renin granules rather than melanosomes, although the latter are not excluded. Expression of HMB-45 in angiomyolipoma has important biologic and diagnostic implications, whether or not it reflects melanocytic differentiation.