This paper presents a feasible method for rapid detection of the interphase nuclei of uncultured amniocytes for chromosomes 18 by using our modified primed in situ labeling (PRINS) technique. A total of 262 independent, uncultured amniotic fluid samples were analysed in a blind fashion before the karyotype was available. In addition, 62 samples were examined by fluorescence in situ hybridization (FISH) for comparison. In more than 95% of the samples PRINS reactions with primer 18cen were successfully induced. Two samples were properly identified and correctly scored as trisomic 18. PRINS reaction could be performed automatically in less than one hour with a programmable thermocycler. Our studies showed that the PRINS technique is simple, rapid and cost-effective. It is as sensitive and specific as FISH; can enhance the accuracy of standard cytogenetic analysis; and allows identification of chromosomes 18 aneuploidies in uncultured amniocytes in significantly less time.