Gallbladder carcinoma producing human chorionic gonadotropin. A primary carcinoma of the gallbladder producing human chorionic gonadotropin (HCG) was encountered in an 83-yr-old Japanese woman, with elevation of HCG/beta-HCG in urine and serum. Remarkable elevation of serum estradiol was an associated finding, with increased HCG. At autopsy, we found that the primary carcinoma of the gallbladder extensively involved the liver. Histologically, the tumor revealed adenosquamous cell carcinoma in the primary site, and moderately to poorly differentiated adenocarcinoma in metastatic foci. Immunohistochemical staining for beta-HCG showed a positive reaction in adenocarcinoma components. This is an extremely rare case of an HCG-producing gallbladder carcinoma, which leads us to speculate that HCG-positive tumor cells may occur due to dedifferentiation. 