Down Syndrome (DS), is the most common chromosomal aneuploidy and also the most common genetic reason of mental retardation. It is generally characterized by an extra chromosome 21. The incidence is 1:1000. Usually, there is no phenotypic difference between complete trisomy 21 and translocation trisomy 21. Two patients were sent to our laboratuary of medical genetics because of clinical features of DS. Conventional cytogenetic techniques were performed for these patients. The karyotypes were found as 46,XY, rob(21)(q10q10) and 46,XX, rob(21)(q10q10). The karyotypes of the parents were normal. The risk of the translocation DS can be estimated by analysing the translocated chromosome in both of the parents. Chromosomal analysis, family history, pedigree, ages and karyotypes of the parents are the basic factors at estimating the risks. In this study, the clinical features and genetic counselling of the translocation trisomy 21 are presented based on the literature.