Introduction: Pentasomy of the X (49,XXXXX) is a rare chromosomal alteration affecting women and was described in 1963 by Kesaree and Wooley. To date, fewer than 30 cases have been reported in the literature. A case of pentasomy of the X chromosome is presented, and by means of molecular biology techniques (microsatellites) the maternal origin of the additional X chromosomes was determined. Clinical case: 28-month-old patient, with proportionate low height, brachycephaly, characteristic fascies, female external genitalia with hypoplastic labia majora, brachylaxis, bilateral fifth finger clinodactyly, right knee luxation, varus deformity. Karyotype was performed in peripheral blood that reported a chromosomal complement 49,XXXXX. Materials and methods: DNA and PCR extraction was performed for the amplification of eight microsatellites or tetra and dinucleotidic STRs located along the X chromosome. The amplified products were analyzed in the ALF EXPRESS sequencer. Haplotype construction and gene dose analysis were performed with the allelic information by determining the area under the curve. Results and discussion: the analysis of the eight STR's performed on the patient and her parents, allowed to establish that the extra X chromosomes correspond to allelic information inherited from the mother. Results and events that have been documented as related to non-disjunction phenomena are analyzed. Conclusion: the origin of the double non-disjunction generated by pentasomia is maternal, where a tetrasomal ovule with four copies of the X chromosome was fertilized with a normal monosomal spermatozoid.