A long-term programme to reduce the incidence of Huntington's Chorea has been established in South Wales, based on a study of all individuals known to be affected or at high risk in a defined population of 1.7 million people. Systematic but non-directive genetic counselling is being given to all adults at risk, accompanied by regular follow-up to provide both further information and practical support where required. The number of observed and projected new cases of the disorder born between 1900 and 1970 has remained almost constant, despite a progressive reduction in birth-rate in the general population during this period. Prospective monitoring of all births in the high-risk population will allow an estimate to be made of future trends and will show whether preventive measures are having any significant effect on the future incidence of the disorder in the population.