Magnesium deficiency has been proposed to be related to the mitral valve prolapse syndrome. However determinations of magnesium concentration in blood serum and hemolysates of red blood cells in this disease revealed divergent results. We investigated 49 patients with the mitral valve prolapse syndrome and 30 healthy individuals, matched by age and gender. The concentration of magnesium was measured in blood plasma and lysates of lymphocytes isolated from venous blood. The magnesium concentration in plasma was similar in both the patients and the healthy controls. By contrast, the magnesium concentration in lysates of lymphocytes was significantly lower (p < 0.03) in the patients with mitral valve prolapse syndrome than in the controls (range: 1.94-10.6 mmol/g of protein in Lymphocyte lysate; median 5.98 and 6.7-9.61 mmol/g of protein; median 7.22). The results obtained suggest that magnesium deficiency may be part of the mechanisms of the mitral valve prolapse syndrome.