Cyclocreatine (CCr), a substrate analogue of creatine kinase (CK), exhibits antitumor activity in vitro and in vivo. To address its mechanism of action, we have examined its effects on tumor cell proliferation, viability, and cell cycle progression. Complete inhibition of proliferation of ME-180 cervical carcinoma cells was observed within 8 h of exposure to CCr and was characterized by an inhibition of progression out of all phases of the cell cycle. This initial effect was partially reversible on drug removal. Increased cytotoxicity was observed after several days of drug exposure and was most specific to cells in S. Previous studies have shown that CCr supports ATP regeneration through the CK system less efficiently than the natural substrate creatine and that CCr is active against tumor cell lines with elevated levels of CK. We propose here that the general inhibition of cell cycle progression reflects an effect of CCr on tumor cell energy availability through CK and that impaired energy homeostasis for several days leads to tumor cell death. Our results point out the unique nature of CCr as an anticancer agent that inhibits progression out of all phases of the cell cycle.