Summary data are presented on 168 D/21 and 131 G/21 translocation trisomies reported to the New York State Chromosome Registry. By combining these data with others from the literature it is estimated that about 59% of D/21 cases are the result of mutation in hte parental generation; the rest are translocations inherited from parental carriers (39% maternal, 3% paternal). The proportion of mutants is about 10% greater for 14/21 cases and significant lower for 13/21 cases. Of G/21 cases 93% are mutant, about 6% of maternal origin, and 1% of paternal origin. All the mutant cases involve 21/21 rearrangements. Estimated mutation rates per 10(5) gametes for translocation trisomies in affected livebirths are 0.1 for 21/13, 0.5 to 0.9 for 21/14, and 1.1 to 1.4 for 21/21. The rates for 21/15 and 21/22 translocation trisomies are probably all conservatively less than 0.1 per 10(5) gametes. Of interchange trisomy Patau's syndrome, about 60% of cases are mutant; the rest are translocations inherited from a parental carrier (about 25% maternal, 15% paternal). The mutant proportion is about 90% for 13/13 cases and about 45% for 13/14 cases. The estimated mutation rates for 13/13 and 13/14 interchange trisomies are each about 0.5 per 10(5) gametes; te rate for 13/15 interchange trisomies is less than 0.1 per 10(5) gametes. A male excess is observed for D/21 (sex ratio = 1.70) and G/21 (sex ratio = 1.38) interchange Down's syndrome, and a female excess for D/13 interchange Patau's syndrome (sex ration = 0.77), trends similar to those seen in the respective 47, trisomies associated with these phenotypes.