- In a survey of Down's syndrome in South Australia, 921 persons, both living and deceased, were identified; 717 individuals with the disorder were living in South Australia. Cytogenetic confirmation of the diagnosis had been made in 774 cases. From 1955 to 1977, the over-all incidence of Down's syndrome at birth was found to be 1.175/1000 live births. The incidence of Down's syndrome was significantly lower over the last five years of this period than for the first 18 years; thus it appears that the incidence of Down's syndrome in South Australia is falling. Analysis of maternal age changes with time has not revealed any changes to the maternal age-specific rates for Down's syndrome, although the rate for mothers aged 25 years or younger appears to be falling. The proportion of Down's syndrome babies born to women aged 35 years or more has decreased from 65.7% for those born before 1950 to 30.4% for those born from 1975 to 1977; similarly, the median maternal age has fallen from 37.12 years to 28.25 years. Regression analyses of maternal age rates for Down's syndrome by single years have produced figures suitable for genetic counselling. A plea is made that Down's syndrome should become a notifiable condition.