Down syndrome (trisomy 21) is the most common chromosomal abnormality in newborns and the most common known cause of mental retardation. There is a well-recognized association between maternal age and the incidence of free trisomy 21. It has been shown that in ~90% of cases with Down syndrome, the trisomy 21 is of maternal origin, in ~5% of paternal origin and ~5% are due to mitotic errors in somatic cells. The determination of the parental and meiotic origin of the extra chromosome in trisomy 21 is of importance in understanding the mechanism of non- disjunction in meiosis and in elucidation of the role of maternal age. The aim of this study was to determine the parental and meiotic origin in Down syndrome patients from the Republic of Macedonia. A total of 29 families, of which 25 with a child with Down syndrome, and four with a prenatally diagnosed fetus with trisomy 21, were studied. The parental origin and meiotic stage of nondisjunction were determined by quantitative fluorescent polymerase chain reaction (QF-PCR) using 10 small tandem repeat (STR) markers (from the centomere to the telomere: D21S215, D21S1256 D21S1410, D21S11/D21S1414, D21S1441, D21S1435, D21S1413, D21S1412, D21S1411 and D21S1416). Pericentromeric markers were used to detect the parental origin and to distinguish between meiosis I errors and meiosis II or mitotic errors. Extra chromosome 21 was due to maternal meiotic I error in 23 (79.3%) cases, maternal meiotic II error in 2 (6.9%) cases and paternal meiotic II error in 4 (13.8%) cases. All four prenatall y diagnosed fetused with trisomy 21 have arisen due to maternal meiotic I error. Our results are in agreement with the data from previous studies and further support the presence of constant chromosome 21 non-disjunction errors in trisomy 21.