Down syndrome is the most frequent autosome aneuploidy in live newborns, caused by trisomy of all or the critical region of chromosome 21. We described a 20 month old boy with Down syndrome phenotype. Full clinical examination was done. Cytogenetic studies for the proband and his parents using GTG- banding technique and Fluorescence Insitu Hybridization technique using LSI 21 and chromosome 9q subtelomere probe were used according to the manufacturer’s instructions. The proband;s karyotype is: 46,XY,der(9)t(9;21)(p24;q22.2) resulted from paternal balanced cryptic translocation. Karyotype of the father is 46,XY, t(9;21)(p24;q22.2) , while mother showed normal Karyotype. FISH analysis using LSI 21 probe and 9q subtelomere probe for the proband’s and father demonstrated the presence of three signals, and two signals respectively. This is the first report describing a paternal cryptic balanced translocation involving chromosome 9 with the generation of aneuploidy contributing to abnormal chromosomal segregation.