Choline kinase (ChoK, E.C. 220.127.116.11) is involved in the synthesis of phosphatidylcholine (PC), and has been found to be increased in human tumors and tumor-derived cell lines. Furthermore, ChoK inhibitors have been reported to show a potent and selective antitumoral activity both in vitro and in vivo. Here, we provide the basis for a rational understanding of the antitumoral activity of ChoK inhibitors. In normal cells, blockage of de novo phosphorylcholine (PCho) synthesis by inhibition of ChoK promotes the dephosphorylation of pRb, resulting in a reversible cell cycle arrest at G0/G1 phase. In contrast, ChoK inhibition in tumor cells renders cells unable to arrest in G0/G1 as manifested by a lack of pRb dephosphorylation. Furthermore, tumor cells specifically suffer a drastic wobble in the metabolism of main membrane lipids PC and sphingomyelin (SM). This lipid disruption results in the enlargement of the intracellular levels of ceramides. As a consequence, normal cells remain unaffected, but tumor cells are promoted to apoptosis. Thus, we provide in this study the rationale for the potential clinical use of ChoK inhibitors.