The genetics and the epidemiology of Down syndrome (DS) was studied in the area which is covered by our registry of congenital malformations. For each of the 398 new DS cases which were ascertained during the period 1979 to 1996 more than 50 factors were studied and compared to those from control infants. The prevalence of DS was 1.66 per 1000; 2.2% of the DS cases were stillbirths and 29.4% were induced abortions. Karyotypes were obtained in 391 cases of which all but 23 were 47,+21;9 were mosaics (2.3%), and 14 had translocations (3.6%). Interchromosomal effect was a question in 7 cases. The most common types of associated malformations were cardiac anomalies (46.2%) and intestinal atresias (6.0%). Seasonality or time/space clusters were not observed in spite of the Chernobyl nuclear accident. No paternal age effect was demonstrated; 5.3% of the mothers of DS had 2 previous spontaneous abortions (controls 3.7% p < 0.05). At birth, the DS infants measured and weighted less and their head circumference was lower than in control infants. Weight of placenta was also lower than in control infants. In this material there were 4.5% of consanguineous marriages (P < 0.01). The pregnancies of the DS children were more often complicated by threatened abortions than in the controls, 3.2% of the mothers of the DS children were diabetic controls (1.7%), although the difference was not statistically significant. For all other factors studied no statistically significant difference with respect to controls could be demonstrated.