Method for determining the severity of Attention-Deficit Hyperactivity Disorder (ADHD), or for determining the effectiveness of a therapy against ADHD, said method comprising the steps of: (i) purifying the genomic DNA from an isolated sample that has been obtained from an ADHD-positive individual or from an individual suspected of being ADHD-positive; (ii) reacting said genomic DNA with bisulfite; (iii) amplifying, preferably by PCR, more preferably by RT-qPCR, at least one portion of the genomic region comprised between chromosomal coordinates 1,392,790 and 1,445,430 of human chromosome 5 comprising at least one CpG dinucleotide or a CpG island, said coordinates being calculated with respect to human genome 19; (iv) measuring the methylation level of at least one CpG dinucleotide or at least one CpG island comprised in the amplified region, wherein said ADHD-positive sample is considered severe if the methylation level measured for said CpG having coordinate 1,444,716 of human chromosome 5 in said sample is higher than the methylation level measured for an ADHD reference sample defined as not severe; wherein said therapy is considered effective if the methylation level measured, in a sample isolated from an ADHD-positive individual following the therapeutic treatment, for said at least one CpG dinucleotide or said at least one CpG island is higher than the methylation level measured in the same sample before said treatment, wherein said CpG or CpG island is selected from at least one CpG having a chromosome coordinate selected from: 1,444,713 and 1,444,692 of human chromosome 5, wherein said therapy against ADHD comprises a cognitive-behavioral therapy and a pharmacological therapy based on the administration of psychostimulant drugs selected from methylphenidate and amphetamine derivatives. (i) purifying the genomic DNA from an isolated sample that has been obtained from an ADHD-positive individual or from an individual suspected of being ADHD-positive; (ii) reacting said genomic DNA with bisulfite; (iii) amplifying, preferably by PCR, more preferably by RT-qPCR, at least one portion of the genomic region comprised between chromosomal coordinates 1,392,790 and 1,445,430 of human chromosome 5 comprising at least one CpG dinucleotide or a CpG island, said coordinates being calculated with respect to human genome 19; (iv) measuring the methylation level of at least one CpG dinucleotide or at least one CpG island comprised in the amplified region, wherein said ADHD-positive sample is considered severe if the methylation level measured for said CpG having coordinate 1,444,716 of human chromosome 5 in said sample is higher than the methylation level measured for an ADHD reference sample defined as not severe; wherein said therapy is considered effective if the methylation level measured, in a sample isolated from an ADHD-positive individual following the therapeutic treatment, for said at least one CpG dinucleotide or said at least one CpG island is higher than the methylation level measured in the same sample before said treatment, wherein said CpG or CpG island is selected from at least one CpG having a chromosome coordinate selected from: 1,444,713 and 1,444,692 of human chromosome 5, wherein said therapy against ADHD comprises a cognitive-behavioral therapy and a pharmacological therapy based on the administration of psychostimulant drugs selected from methylphenidate and amphetamine derivatives.