The study was carried out on 48 patients (26 male and 22 female), aged 52-76 diagnosed as having cancer metastases to the liver. The serum level of magnesium in the studied group and also in two other groups (the first consisting of 48 blood donors and the second of 50 patients suffering from cancer of a non-disseminated nature) was measured by the flame atomic absorption spectrophotometric method. The results of the study showed that cancer metastases to the liver significantly decrease the mean level of magnesium in serum compared to the other two groups. It was shown that a magnesium level below 12.76 micrograms/ml in cancer patients increased the risk of cancer metastases to the liver.