Magnesium is fundamental to the existence of life. The consequence of altered magnesium homeostasis may be magnesium deficiency. It is well known that magnesium plays a role in tumour biology such as carcinogenesis, angiogenesis and tumour progression. In the field of gastrointestinal cancer surgery of the clinical importance, magnesium has not been specifically studied. Therefore, the aim of our study was to evaluate changes of magnesium concentrations in patients operated due to a small intestine or colorectal cancer parenterally nurtured in comparison with a group of patients submitted to surgical interventions due to gastrointestinal cancer but receiving standard nutrition after the operation. The study group involved 78 patients operated on for gastrointestinal cancer, who were divided into 3 groups: C – patients operated due to different types of alimentary tract cancers who were provided with normal feeding after the operation, I – patients operated due to colorectal cancer who were given TPN after the operation, II – patients operated due to small intestine cancer who were given TPN after the operation. Three measurements were performed in control group (C): the 1st measurement – a day before operation, the 2nd measurement – on the third day after the operation and the 3rd measurement – on the fifth day after the operation. In the group of patients receiving TPN, three measurements were performed as well: the 1st measurement – a day before operation, the 2nd measurement – on the third day after applying TPN and the 3rd measurement – on the fifth day after applying TPN. Our studies revealed that application of TPN, containing magnesium, in patients operated both due to colorectal cancer and small intestine cancer prevented decrease in the blood serum concentration of that element below the reference norm, which occurred in patients receiving standard diet.