The authors describe a population-based birth defects registry, started in 1976. The system surveys about 85,000 births per year, occurring in 140 maternity units and representing more than ten per cent of all the births in France. Monitoring first covered the Rhône-Alpes region, then was extended to the Auvergne region in 1983 and to the Jura district in 1985. The method of investigation was "multi-source", because any doctor in the zone covered was in a position to notify a malformation to the registry. (497 obstetricians, pediatricians, pediatric surgeons, fetopathologists, geneticists and cytogeneticists). Malformations were coded with a specific terminal elaborated in the registry (1,600 items). The mothers' exposures to drugs during the first trimester of pregnancy were coded by trade names. The registry is a full member of the International Clearinghouse for Birth Defects Monitoring Systems, an international organisation now including in this group 25 regional or national birth defects registries and covering more than 3 million births per year. The 1986 results of monitoring birth defects in the described registry are given as examples. Within the eleven years (1976-86), 15,000 cases of malformations were registered, and two clusters have been detected and followed-up: femoral aplasia/hypoplasia in 1980-81 and oesophageal atresia in 1984. No cause was found for these "epidemics". The strong association between in utero exposure to valproic acid with spina bifida is the main result of the activities of the registry since its creation.