The aim of my work was the answer to the following questions: how often does the deficiency of magnesium, copper, zinc, calcium, iron occur among hyperactive children in comparison with healthy children, deficiency of which of the considered bioelements is the most frequent, what is the effect of supplementation of deficit element on hyperactivity and does it depend on other certain disorders that coexist with hyperactivity? In a process of establishing the subject diagnosis I have followed the DSM IV criteria recognizing ADHD among examined ones. I have determined the deficiency of magnesium, copper, zinc, calcium, iron in the group of 116 children with diagnosed ADHD. Consequently, as a result, I have found out that shortage of above-mentioned bioelements occurs more often among hyperactive children than among those being healthy, and deficiency of magnesium is the most frequent in this respect. Further, I have divided the group of 110 children with magnesium deficiency into two groups according to the other mental disorders that coexist with ADHD: 1) the group where hyperactivity coexists with disorders typical for developmental age such as enuresis, tics, separation anxiety, stuttering, selective mutism (63 children); 2) the group where hyperactivity coexists with disruptive behaviour disorders: conduct disorder and oppositional defiant disorder (47 children). The content of magnesium, copper, zinc, calcium, iron has been determined respectively in blood (serum and red cells) and in hair by atomic absorption spectroscopy method in both groups accordingly. At the same time, the hyperactivity tests were carried out using Conner's Rating Scales for Parents and Teachers, Wender's Scale as well as Quotient of Development to Freedom from Distractibility. During the statistical analysis the inparametric tests have been used taking as a significance level p < 0.05. On the ground of obtained findings I have not stated any significant differences in bioelements content among hyperactive children in relation to other coexisting disorders, except for zinc. The zinc content in hair was higher among children with ADHD and disruptive behaviour disorder. The assessment of hyperactivity indicated the remarkably higher coefficient among children with coexisting behaviour disorders as compared to hyperactive children among whom, additionally, disorders typical for developmental age have occurred. The analysis of influence exerted by magnesium supplementation on hyperactivity has been carried out in the group of total 75 children with ADHD jointly with magnesium deficiency. The group of 50 children actually tested, apart from standard treatment have received the specified doses of magnesium preparations for 6 months on regular basis. The group of 25 children was left with standard treatment without additional magnesium. In both above-mentioned groups the content of bioelements and respectively ADHD level have been determined just before and after the test. The obtained results have clearly disclosed significant increase of magnesium, zinc, calcium content (Tab. 1) and respectively essential decrease of hyperactivity in the group of children treated with magnesium. At the same time, however, among the children given standard treatment without magnesium, hyperactivity has intensified (Tab. 3, 4). The findings herein presented indicate that it is necessary to take into consideration a possible bioelements deficiency among children with ADHD. Consequently, the accomplished study proves that there is a need of magnesium supplementation in ADHD children irrespectively of other mental disorders. The supplementation of that kind of magnesium supplementation together with standard traditional mode of treatment gives us the opportunity to extend the methods of therapy of ADHD children who are the "children of the risk" in connection with their educational, emotional and social problems.