A case of a rare anomaly of the human karyotype--ring chromosome 21--is presented. By analysing the prenatal amniotic fluid of a 38-year-old woman asking genetic advice, primarily because of a cystic fibrosis burdening her first pregnancy, the authors discovered this chromosomal anomaly and made the karyotype of both parents which showed that mother was a carrier of the same anomaly as observed in her unborn child. The karyotyping of her second 7-year-old child also revealed ring chromosome 21, identical with that in its mother and unborn sister.