Examination of data from the Glasgow Registry of Congenital Anomalies indicated that 184 infants with Down's syndrome were born (live or still) to mothers residing in the Greater Glasgow Health Board area between 1974 and 1986 inclusive. This represents a period prevalence of 1.1 per 1000 total births. Despite a strongly positive correlation between prevalence and maternal age, most of the Down's syndrome infants were born to mothers aged under 35 years. There was no evidence either of a recent decline in the annual prevalence rate or of a changing pattern of risk in relation to maternal age. Antenatal diagnosis resulted in the termination of less than a tenth of all Down's syndrome pregnancies. These findings point to a need for further aetiological research, for continued epidemiological monitoring, for an improvement in the relatively low uptake of amniocentesis by older mothers, and for the development of a screening test which can be offered to the entire pregnant population.