BACKGROUND: The prognosis for patients with unresectable advanced hepatocellular carcinoma (HCC) is poor. Miriplatin is a hydrophobic platinum compound that has a long retention time in lesions after transarterial chemoembolization (TACE). We investigated anti-tumor activity of miriplatin combined with irradiation on HCC cells, and its underlying mechanism of apoptosis. We also analyzed the effectiveness of miriplatin-TACE and radiotherapy for locally advanced HCC. METHODS: Human HCC cell lines HepG2 and HuH-7 were treated with DPC (active form of miriplatin) and radiation, and synergy was evaluated using a combination index (CI). Apoptosis-related proteins and cell cycles were analyzed by western blotting and flowcytometry. We retrospectively analyzed treatment outcomes in 10 unresectable HCC patients with vascular/bile duct invasion treated with miriplatin-TACE and radiotherapy. RESULTS: DPC or X-ray irradiation decreased cell viability dose-dependently. DPC plus irradiation decreased cell viability synergistically in both cell lines (CI < 1, respectively). Cleaved PARP expression was induced much more strongly by DPC plus irradiation than by each treatment alone. Expression of p53 up-regulated modulator of apoptosis (PUMA) was significantly induced by the combination, and knockdown of PUMA with siRNA significantly decreased apoptosis in both cell lines. DPC plus irradiation caused sub-G1, G2/M, and S phase cell arrest in those cells. The combination of miriplatin-TACE and radiotherapy showed a high response rate for patients with locally advanced HCC despite small number of patients. CONCLUSIONS: Miriplatin plus irradiation had synergistic anti-tumor activity on HCC cells through PUMA-mediated apoptosis and cell cycle arrest. This combination may possibly be effective in treating locally advanced HCC.