In New York State exclusive of New York City the standardised rate of Down syndrome in livebirths to women 35 and over reported on birth certificates increased 30% in 1970 and later years, compared to the rate in 1965 and 1969. In contrast, there was no change in those under 35. A separate investigation revealed that selective temporal differences in reporting of cases born to older mother did not account for the change. A similar increase in rates of cases born to older but not younger mothers in 1970 and later years has also been reported from Manitoba. These observations may be pertinent to apparent discrepancies between livebirth rates of Down syndrome and those reported at amniocentesis.