Objective To explore the potential application of molecular methodology in the detection of numerical chromosome aberration for clinical diagnosis and prenatal diagnosis of Down’s syndrome Methods The primed in situ labelling (PRINS) procedure was carried out by in situ annealing of specific oligonucleotide primers to chromosome 13, 16, 18, 21, X and Y, respectively, followed by primer elongation by a Taq polymerase in the presence of labelled nucleotides Detection of the labelling sites was performed by immunocytochemistry and conventional fluorescence microscopy Results Under the stringent annealing temperature, chromosomes 13, 16, 18, X and Y were specifically labelled at centromeres and the procedure was carried out successfully on interphase nuclei as well as on metaphase spreads with easily scorable signals In 33 cases of uncultured peripheral blood lymphocytes and 11 cases of uncultured amniocytes tested, two fluorescence signals were shown on more than 87 6% interphase nuclei when chromosomes 13, 16, 18 were investigated Sex chromosomes were correctly detected in the same way Blind tests on peripheral blood lymphocytes from 14 cases of normal individuals and 12 cases of Down’s syndrome patients and on 9 cases of amniocytes showed that when chromosome 21 was detected by PRINS, two fluorescence spots as positive signals were visible on 89 3% normal nuclei and three spots on 88 8% trisomic nuclei The above results were fully compatible with karyotypic analysis Conclusion PRINS provides a rapid and efficient method for the clinical diagnosis or prenatal diagnosis of trisomy 21