The chromosome findings in a series of 173 Down’s syndrome or presumptive Down’s syndrome index patients, and a further 3 5 pairs of parents of Down’s syndrome index cases, are given. One hundred and forty-four index cases had primary G21 trisomy; six were interchange trisomics, six were mosaics, and one was a chromatin positive male with XXY sex chromosomes and primary G21 trisomy. Sixteen index patients whose diagnosis could not be clinically confirmed, were found to have normal chromosomes. The 35 pairs of parents all had normal chromosomes. In seven families (three index patients with primary G21 trisomy, two with normal chromosomes and two index patients not studied) minor morphological chromosome anomalies were observed. The relevance of this data to the causal association of Down’s syndrome and G21 trisomy, the frequency of mosaicism, double trisomy and association with other chromosome anomalies, is discussed.