Ritonavir, a protease inhibitor, has been successfully applied in the treatment of HIV infection. Reports of dramatic improvement of AIDS-related cancers, such as primary central system lymphoma after radiation therapy as well as Kaposi's sarcoma, led to the recent discovery of the "non viral" antitumor activity of HIV protease inhibitors. This study was designed to detect the antitumor effect of Ritonavir when combined with ionizing radiation both in vitro and in vivo in the HEP-2 head and neck carcinoma model. Inhibition of tumor growth was observed when mice were treated with Ritonavir alone and this effect was enhanced when combined with ionizing radiation. No adverse effect or significant toxicity in the hosts' body weights was seen between the different treatment and control groups throughout the experiments. A marked antiproliferation effect of the combination was observed in vitro. A marked reduction of angiogenesis was detected within the tumor sections from the Ritonavir combined with irradiation group as compared with the Ritonavir or irradiation alone groups. Western blot analysis showed that apoptosis was induced by an increased expression of Bax and decreased expression of Bcl-2 after treatment with Ritonavir and ionizing radiation. Thus, the antitumor effect of the latter combination is associated with the enhancement of radiation-induced apoptosis and inhibition of angiogenesis. These data suggested that Ritonavir could clinically improve the tumor response to radiation therapy, especially in head and neck carcinoma.