Trisomy 21 (Down syndrome) is the most common autosomal aneuploidy in humans. Diagnosis of chromosomal abnormalities is usually performed by conventional cytogenetic analysis, but this procedure is lengthy and requires great technical expertise. In the past few years an alternative molecular method based on the fluorescent amplification of short tandem repeats markers (STRs) by polymerase chain reaction (PCR) for the rapid diagnosis of trisomies has been developed.  The purpose of this study was to evaluate the efficiency of quantitative fluorescent polymerase chain reaction (QF-PCR) tehnique in detection of trisomies, particularly for the  postnatal detection of trisomy 21. Ten karyotyped blood samples from 21 trisomic patients and five blood samples from normal individuals have been analysed by amplification of 2 STR markers specific for chromosome 21 (D21S11 and Penta D) followed by detection of fluorescent PCR products by capillary electrophoresis. Trisomy 21 was confirmed by both cytogenetic and quantitative fluorescent PCR methods.