An estimate of prevalence of mongolism at birth in Northern Ireland (1960-69 gave a frequency of 2.23 per 1000 livebirths. This figure is considerably higher than from other published sources within the United Kingdom. The estimate was derived by applying mortality experience of a sample of mongols over a recent time period. Mortality among mongols born to older mothers was after one year almost fifty per cent whilst those born to younger mothers was twenty-three per cent. These findings have implications for the viability of future screening programmes.