Chemotherapy is effective against a wide variety of tumor cells, although its use is limited by side effects. In vitro experiments and phase I and II trials have shown that phytochemicals such as perillyl alcohol (P-OH) have antitumor effects. Pentoxifylline (PTX), a synthetic methylxanthine used mainly to treat pathologies associated with hematological diseases, sensitizes tumor cells to chemotherapy. The aim of this study was to determine whether PTX amplifies the antitumor effects of P-OH in U937 human myelomonocytic leukemia cells. Apoptosis was measured by the loss of mitochondrial membrane potential determined by flow cytometry using dihexyloxacarbocyanine iodide (DiOC6) and propidium iodide. Bcl-2 and Bax protein expression was also assessed by Western blot analysis. P-OH and PTX induced loss of the mitochondrial membrane potential in U937 cells in vitro. Culturing the cells in the presence of both compounds caused a significant increase (p < 0.001) in apoptosis and expression of anti-apoptotic Bcl-2 and pro-apoptotic Bax proteins. However, despite their coexistence, Bax expression prevailed in our experiments. These data suggest that the effects of PTX might be attributable to changes in the mitochondrial membrane potential. PTX sensitizes tumor cells to the anti-neoplastic action of P-OH. These observations may have clinical relevance in the treatment of cancer patients.