There are at least 240 million heterozygotes for the haemoglobinopathies throughout the world, and at least 200 000 lethally affected homozygotes are born annually, about half with thalassaemia and half with sickle cell anaemia. These diseases are already making important demands on health resources in developing countries, and with further progress in primary health care, they will become a major public health problem. This has already happened in the Mediterranean areas where thalassaemias are predominant, and is now beginning in parts of south-east Asia. Important new developments in the treatment and community control of the haemoglobinopathies now make it advisable for all countries where they are endemic, regardless of their present state of development, to evaluate the present and potential problems presented by these diseases.Although effective management of thalassaemia major, involving intensive treatment with blood and iron-chelating agents, is too expensive for most developing countries, community control by prospective heterozygote detection, education, and fetal diagnosis has now been successfully applied in some European and Mediterranean areas. Within three years of starting these programmes, the birth rate of infants with thalassaemia major had fallen by 50-80%. The relatively low cost of setting up and running the control programmes has already been greatly outweighed by the financial and social benefits of the reduced thalassaemia birth rate. Detailed guidelines are provided for community control programmes that could be incorporated into the health care service of many developing countries.In sickle cell disease, which varies in severity with both genetic and environmental factors, the commonest problem is death in early childhood from overwhelming infections. A major effort should be put into heterozygote screening and neonatal diagnosis to permit earlier identification and protection of affected individuals. This will also provide a good basis for a prevention programme if more acceptable methodology, such as first trimester fetal diagnosis, becomes available. Heterozygote detection and counselling, and education in management of homozygotes should be integrated into the primary care system of countries with a high incidence of sickle cell disease.