Weatherall J A C (EUROCAT Central Register and Department of Epidemiology, Catholic University of Louvain, School of Public Health, Clos Chapelle-aux-Champs, 30, 1200 Brussels, Belgium), de Wals P and Lechat M F. Evaluation of information systems for the surveillance of congenital malformations. International Journal of Epidemiology1984, 13: 193–196. Many systems for the recording of congenital malformations in infants have been developed following the thalidomide epidemic of 1958 to 1962. Systems established for monitoring increases in rates of malformations have been of three main types: (1) prospective recording of information about all pregnancies; (2) the recording of malformations observed at birth; (3) the registration of children found to be malformed at birth or at any time after birth. The latter type involves many sources of information. International collaboration in monitoring has led to the establishment of the International Clearinghouse for Birth Defects Monitoring Systems and the EEC Concerted Action Project EUROCAT. The advantages of these two projects are discussed. It is shown that results from population based registers such as are included in EUROCAT can be used to validate the results of systems which carry out monitoring at birth. EUROCAT Central Register and Department of Epidemiology, Catholic University of Louvain. School of Public Health. EPID 30.34, Clos Chapelle-aux-Champs. 30, 1200 Brussels. Belgium.