A study of Down's syndrome in Lothian for the years 1978-1989 revealed a significantly higher than expected incidence of cases in 1987. The increased incidence was particularly significant among women aged 35 years or more. The increase could not be attributed to demographic changes on birth rates or to the age distribution of the women concerned. A temporal association between the cluster of cases and the events at Chernobyl in April 1986 was demonstrated. However, according to current understanding, there is no biologically plausible explanation to link the 2 events, given the reported levels of radioactive fallout in the Lothian region, resulting from the Chernobyl disaster. An explanation for the increased incidence of Down's syndrome in 1987 remains outstanding.