During a 7-year-period (1980-1986) trisomy 21 was confirmed in 635 cases (257 males and 278 females). There were 611 cases of trisomy 21 (96.2%), 12 of different translocations (1.9%), 9 of mosaicism (1.4%), and 3 with nonclassical karyotypes (0.5%). The frequency of chromosome aberrations in our study is compared to that of major world-wide cytogenetic surveys comprising 17,738 Down syndrome cases. These surveys showed that regular trisomy 21 constitutes 92.9%, translocations 4.3%, mosaicism 2.2%, and nonclassical karyotypes 0.5%.