Digestive system neoplasms pose a serious health problem both in Poland and abroad. Neoplasms are frequently considered to be caused by impaired homeostasis in the human body. Development of neoplasms may be linked to disturbances in concentration of elements, including magnesium as a major intracellular cation. The objective of this study was to evaluate the concentration of magnesium in plasma and tissue samples taken from patients suffering from neoplasms of the stomach or the large intestine. The study involved 35 patients, including 20 affected by stomach cancer and 15 suffering from large intestine cancer. The patients were in the age rage of 36-77. The material included blood samples taken from patients before and seven days after surgery, as well as samples of cancerous and healthy tissues. The colorimetric method with a Genesis spectrophotometer was used for determination of magnesium concentration. A statistically significant difference was observed between plasma magnesium concentration in patients affected by stomach cancer and the normal range. Elevated values of magnesium concentration measured on the seventh day after the procedure as compared to the concentration before the procedure was noted, however, the difference was statistically insignificant. No significant differences were observed in magnesium concentration measured before and after the procedure, or in comparison to the normal range in patients with large intestine cancer. Determination of tissue magnesium showed that magnesium concentration was higher in cancerous than in healthy tissue. Obtained results demonstrate that magnesium homeostasis is impaired in patients, which may be important in the pathoghenesis of digestive system neoplasms.