Down syndrome is the most common autosomal syndrome in man. The incidence of trisomy 21 due to translocation is about 3.5-5%. Translocations are usually centric fusions between a 21 and a D group (54.2%) or a G group (40.9%) chromosome. Since the short arm of 21 carries no phenotypically active genes, even if the short arm of 21 is lost, usually there is no imbalance with clinical significance. Rare translocations in Down syndrome may also occur between a 21 and other chromosomes (4.9%). We experienced a case of unusual (1q;21q) translocation Down syndrome inherited from a t (1q;21q) balanced carrier mother. The segmental trisomy was confirmed by FISH analysis. This is the first case of a familial Down syndrome with t (1q;21q) in Korea.