The results of several experimental and epidemiological studies have shown an inverse correlation between Mg status and the risk of some cancers. However, relationship between magnesium and cancer is complex. The aim of our work was to examine the precise effect of Mg deficiency on transplantable mouse tumor growth and metastasis. The results obtained indicate a significant retardation of primary tumor growth (up to 70%) in mice receiving Mg-deficient diet. However, Mg repletion caused in these mice significant increase of primary tumor burden. Analysis of cell cycle distribution showed a reduced percentage of cells in the S phase and an increase of cells in the G(0)/G(1) phase of the cell cycle in LLC tumors caused by Mg deficiency. This is in agreement with the effect of low Mg level on cell growth observed in vitro. Interestingly, in mice inoculated with LLC cells and receiving low-magnesium diet, a higher metastatic potential was observed as compared to control mice. In conclusion, our results demonstrate a direct role of magnesium in tumor growth and also point at deleterious effect of low magnesium status on tumor metastasis.