[Epidemiological aspects of trisomy 21].

The authors analyse cytogenetic and epidemiological data of 4760 cases of trisomy 21 found in 22 European Centres of Genetics. This cytogenetic study is dealing with the frequency of regular free trisomies and those in mosaics and of trisomies by translocation. The frequency of the different types of translocation has been established thanks to an identification of the chromosomes through marking techniques. The risks and causes of recurrence are discussed. From the epidemiological aspect, the role of the mother's age was the subject of a special study which confirms the bimodal distribution of the trisomy 21. This distribution is discussed in association with the other epidemiological factors: X-ray, virus, auto-immunity and gynaecological disorders.