Patients with chronic heart failure suffer more frequently, due to cumulation of factors leading to impaired magnesium (Mg) metabolism, from disorders of its concentration in serum and urine. Hypomagnesaemia has a marked (statistically significant) negative impact on the one-the two-year survival of patients. The author examined 57 patients with chronic heart failure. He did not detect hypomagnesaemia in any of them (Mg levels below 0.7 mmol/l). In two instances he found reduced urinary levels of Mg (less than 1 mmol/24 hours). Surprising is the more frequent occurrence of hypermagnesaemia (Mg above 1.0 mmol/l) which has also an adverse (statistically significant) prognostic effect on the one- and two-year survival of these patients. In the discussion the author deals with the effect of individual factors and drugs on Mg concentrations in serum and urine.