This study demonstrated that both congenital cardiovascular defects and the cytogenetic form of trisomy 21 have considerable effects on the chronology of the first dentition. The delay in the eruption of the deciduous teeth was even greater in children with congenital heart defects than in those with free trisomy 21. So-called mosaic-types of Down's syndrome, on the other hand, were associated with almost regular times of eruption.