Among 257 patients with clinical diagnosis of Down's syndrome, 56.4% of male gender, in whom cytogenetic studies were performed, 14 (5.4%) had normal karyotypes and 243 (94.6%) had 21 trisomy. Of these last, 225 (92.6%) had free 21 trisomy, 10 (4.1%) showed mosaics, 8 (3.3%) had translocations. Average maternal age of this sample was significantly higher than that of patients attending the same maternity wards (32.14 vs. 24.85 years) and 41% of Down syndrome's patients came from mothers aged 36 years or more, even though only 9.7% of this country's deliveries proceed from women of that age group. As to seasonal occurrence, the proportions of births which happened in summer almost doubled that of winter (33.1% vs. 16.9%). The frequency of characteristic clinical signs of Down's syndrome was somewhat different than that described for patients from some other countries, for instance: epicanthus, short neck and widening of the space between 1st. and 2nd. toe were more frequent, while Brushfield's spots, depressed occiput, dental anomalies, heart malformations and fissured tongue seemed less frequent. The importance of cytogenetic studies for diagnosis and genetic counseling is stressed.