A quantitative fluorescence polymerase chain reaction (QF-PCR) technique based on the determination of triple-dose chromosome-specific short tandem repeats (STR) has been recently developed for the prenatal diagnosis of numeral abnormalities of chromosomes 21, 18, 13 and X and Y. This investigation examined 55 blood samples from healthy donors, 17 amniotic fluid samples, 27 blood samples from children with regular trisomy 21, 1 sample with a translocation variant of Down's syndrome, and 3 samples with triploidy. The heterozygosity of 4 STR markers specific for chromosome 21 was determined, which were used in QF-PCR to detect Down's syndrome.