The mosác of trisomy 21 is characterized by the presence of cell line with extrachromosome 21 and anormd cell line in one individud. The distribution of trisomic line may differ in different tissue and change during the life. The presence of normd cell line diminishes the Down syndrome phenotype. Since 1981 until the year 2003 the aithors exanined 283 children with Down syndrome, 21 of them having the mosác form. In order to determine the change of the ratio of trisomic and normd cell line in lymphocytes the aithors examined the group of 10 patients with the time lapse of 10 and more years from the fist cytogenetic exanination. The age of the patients was 10 - 22 years, the sex ratio being 1:1. The repeated cytogenetic exanination confirmed changes in the ratio of the cell lineš in dl individuds. In the patients with trisomic line 13% - 60% there was adecrease up to 3%. In patients, who displayed 74% - 96% proportion of trisomic line, aithors observed the increase of trisomic line and mosác was abolished and 100% trisomy developed in one patient. In evduating the phenotype the aithors noticed awide variability of symptoms from anormá phenotype to the typicd Down syndrome. However, phenotype of the patients weakly correlated with the ratio of cell lineš in lymphocytes. The correlation with the evolution of mentd function was not confirmed at dl. Prognosis of the development in children with the mosác form of trisomy 21 is more fayorable that in trisomy 21, but the ratio of the cell lineš in lymphocytes does not make it possible to predict further development of the child.

        Key words: mosácism, mosác of trisomy 21, mosác form of Down syndrome, cell line