PTEN mutations are frequently found in malignant glioma and can result in activated phosphatidylinositol-3-kinase/Akt survival signaling associated with resistance to radiotherapy. Strategies to interfere with aberrant PI3K/Akt activity are therefore being developed to improve the therapeutic efficacy of radiotherapy in patients with malignant glioma. The methylxanthine caffeine has been described as a PI3K inhibitor and is also known to sensitize cells to ionizing radiation. However, a direct association between these two caffeine-mediated effects has not been reported yet. Therefore, we asked whether caffeine or its derivative pentoxifylline differentially affect the radiosensitivity of malignant gliomas with different PTEN status. As models, we used the radiosensitive EA14 malignant glioma cell line containing wild-type PTEN and the radioresistant U87MG malignant glioma cell line harboring mutant PTEN. Our study revealed that caffeine and pentoxifylline radiosensitized PTEN-deficient but not PTEN-proficient glioma cells. Radiosensitization of PTEN-deficient U87MG cells by caffeine was significantly correlated with the activation of the G(1) DNA damage checkpoint that occurred independently of de novo synthesis of p53 and p21. The p53 independency was also confirmed by a significant caffeine-mediated radiosensitization of the glioma cell lines T98G and U373MG that are deficient for both PTEN and p53. Furthermore, caffeine-mediated radiosensitization was associated with the inhibition of Akt hyperphosphorylation in PTEN-deficient cells to a level comparable with PTEN-proficient cells. Our data suggest that the methylxanthine caffeine or its derivative pentoxifylline are promising candidate drugs for the radiosensitization of glioma cells particularly with PTEN mutations.