A right renal pelvic mass in a 72-year-old man was resected. The histologic appearance of the tumor was a mixture of tubular adenocarcinoma cells and hepatoid neoplastic cells, and there was a resemblance to hepatoid adenocarcinoma. The intraoperative level of serum alpha-fetoprotein (AFP) was calculated to be 2246 ng/ml, and the postoperative level ranges from 183.6 to 285.6 ng/ml. Lectin binding assays showed that the serum AFP was the hepatic carcinoma type. In a hepatoid portion, an iron-negative, brown to green pigment was positive for bile. Alpha-fetoprotein was immunohistochemically evident in the neoplastic cells. In addition to the hepatic differentiation, the tumor had differentiated into intestinal absorptive or pancreatobiliary tract cells, as deduced from the frequent presence of spicular bodies, a unique light microscopic feature equivalent to microvilli with an actin core. The hepatoid adenocarcinoma is a distinct type of AFP-producing carcinoma present in the organs with epithelium of endodermal origin. Hepatoid adenocarcinoma in the renal pelvis may arise from a metaplasia of neoplastic mesonephric cells into endodermal cells.