Magnesium deficiency has been suggested to be related to the mitral valve prolapse syndrome (MVPS). The aim of the present study was to analyse the concentration of magnesium in blood plasma of patients (pts) with MVPS. In the group of 80 subjects, including 50 pts with MVPS and 30 healthy people matched for age and gender, who comprised the control group (CG), concentration of magnesium in blood plasma was estimated. Magnesium levels were measured by atomic absorption spectrophotometry. Mean concentration of magnesium cation in plasma in pts with MVPS was 0.74 +/- 0.12 mmol/l (range 0.47-1.02 mmol/l). It was only 1.02% lower than in the CG (x = 0.76 +/- 0.07 mmol/l; range 0.67-0.97 mmol/l). However evaluation of the magnesium concentration in blood plasma did not prove magnesium deficiency in the mitral valve prolapse syndrome. Moreover the study revealed that histograms of magnesium concentration values in both investigated groups were divergent.