Incidence rates derived from prospective studies of 120,000 newborns were applied to live births and induced abortions to estimate the trend of Down''s syndrome in New York State, USA, after liberalization of the abortion law in 1970. Women aged 35 yr or older, at higher risk of Down''s syndrome births, sought terminations of their pregnancies more frequently than did younger women. The estimated number of newborns with Down''s syndrome in the state declined 20% from 1971 to 1975. In New York City, more pregnancies in the high-risk age range were aborted than were carried to term. Abortion reform may have made a significant contribution to the reduction of severe mental retardation.