The aim of this study was to present the first case with Down syndrome in conjunction with de novo isochromosomes of both short and long arm of the chromosome 21. Cytogenetics, molecular cytogenetics and molecular genetic analysis were performed on chorionic villus sampling at 12 weeks of gestation of a 42-years-old pregnant woman. According to cytogenetics, molecular cytogenetics and molecular genetic analysis the karyotype was designated as: 47,XY,i(21) (qter --> q10::q10 --> qter),+i(21) (pter --> p10::p10 --> 10pter).ish i(21)(qter --> q10::q10 --> qter)(CEP13/21+,WCP21+),+i(21) (pter --> p10::p10 --> pter)(CEP13/21+,WCP21+). Quantitative Fluorescent Polymerase Chain Reaction (QF-PCR) analysis revealed that isochromosome 21q was maternal in origin. After the detailed genetic counseling, the family decided termination of the pregnancy. This is the first report of co-existence of an isochromosome 21p and an isochromosome 21q in a case with Down syndrome. Our case shows the importance of the molecular cytogenetics and molecular genetic analysis in cases with isochromosomes of the acrocentric chromosomes and supernumerary marker chromosomes regarding to highlight of the formation mechanisms of co-existence of these two rearrangements.