Objective. Basic studies indicate magnesium deficiency as one of the important, but often neglected, risk factors aggravating the course of borderline disorders (BD). A clinical verification of this notion has been conducted. Materials and methods. Authors studied 62 patients with BD, aged 25—65 years, of inpatient and outpatient settings. Contents of magnesium and other blood electrolytes were determined. Results. Authors found an extremely high prevalence of very low levels of magnesium (Mg) in erythrocytes (<0.3 mmol/l) in patients with BD compared to controls (patients without BD, Mg (er.) 1.62±0.48 mmol/l). It has been shown that low Mg levels in the plasma and red blood cells are associated with a significantly increased risk of the following diagnoses: F07 «Personality and behavioral disorder due to brain disease, damage and dysfunction» (p<0.0016), F21 «Schizotypal disorder» (p<0.0005) and F34 «Persistent mood [affective] disorders» (p<0.0001). The use of Magne B6 Forte (4 tablets/day, 30 days, then 2 tablets/day for 1 year) resulted in a significant increase in the Mg levels in the plasma and erythrocytes, the compensation of anxiety and depressive symptoms, improvement of sleep and general health of the patients, reduced consumption of antidepressants (by 30%). Conclusions. Administration of the drugs based on organic salts of magnesium per os improves the condition of patients and reduces their need in pharmacotherapy.