This study assesses the therapeutic efficacy of radiolabeled iodized oil on a patient with hepatocellular carcinoma (HCC). An iodized oil, such as Lipiodol or Ethiodol (Savage Laboratories, Melville, NY), was retained selectively in the tumor vessels of large tumors as well as in the daughter tumors of HCC for long periods of time following intra-arterial injection into the hepatic artery proper. A small fraction of the stable iodine (1 pg of I-127) of the 37% iodine by weight in Ethiodol was replaced by the I-131 with 100% efficiency. A patient with HCC was injected with I-131 Ethiodol into the hepatic artery. Sequential imaging of organs such as the liver, lung, stomach, and thyroid over an eight-day period demonstrated a high tumor-to-normal-liver ratio and a negligible amount of radioactivity in these organs. These findings indicate that I-131 Ethiodol, or Ethiodol labeled with other pure beta emitters, such as Y-90 or P-32, will be effective delivering a high internal radiation dose to HCC with a small radiation effect to normal tissues. To evaluate its potential as a radiotherapeutic agent for HCC, the kinetics, biodistribution, determination of absolute activity in the tumor following intra-arterial injection of I-131 Ethiodol will be studied in the future. At the same time, an effort will be made to label Ethiodol with Y-90 and P-32.