Two cases of hepatoid adenocarcinomas were studied with an in situ hybridization technique (ISH) using a RNA probe for human albumin mRNA. In case 1 the urinary bladder of a 67-year-old woman was affected; in case 2 the tumour was located in the gastric antrum of an 80-year-old woman. In neither case had alpha fetoprotein (AFP) been determined preoperatively. Histologically these cases showed adenocarcinomatous features intermingled with hepatoid areas. These latter areas were characterized by cords of polygonal cells, each with an oval nucleus and prominent nucleoli, separated by a fine network of sinusoids. In the hepatoid areas the immunohistochemical profile was similar to that observed in hepatocellular carcinomas, in that the tumour cells were positive with AFP, alpha-1-antitrypsin (A1AAT) and albumin antisera and there was a canalicular type of reactivity with polyclonal anti-CEA (pCEA) antibody. ISH revealed albumin mRNA in virtually all hepatoid cells in case 1, and in about 50% of those in case 2. In addition, in case 2 occasional cells in the adenocarcinomatous areas showed albumin transcripts of ISH. Our findings confirm that ISH for albumin mRNA probe is a valuable method of establishing hepatocellular differentiation, and that hepatoid adenocarcinomas are tumours with true extrahepatic hepatocellular differentiation.