The current study was undertaken to reevaluate the histogenesis of adenomatoid tumor by an immunoperoxidase method of detecting factor VIII related antigen, a tissue specific marker for vascular endothelium. Three of eight adenomatoid tumors showed diffuse cytoplasmic staining of the cells lining the lumina. When these results were correlated with the light and electron microscopy, it was found that there were two distinct groups of tumors. The vascular adenomatoid tumors stained positively for factor VIII related antigen and did not have tumor cells between tubules. The mesothelial adenomatoid tumor did not stain for factor VIII related antigen, had individual and solid nests of tumor cells between the tubules and had mesothelial characteristics on electron microscopy. These results strongly suggest that adenomatoid tumors can be subdivided into tumors of either mesothelial or vascular endothelial origin, and that these two groups can be distinguished on light microscopy.