A population-based study of 7,049 index patients with multiple congenital abnormalities (MCA) born in Hungary during [1973][1974][1975][1976][1977][1978][1979][1980][1981][1982] was organized by the Hungarian Center for Congenital Anomaly Control. All clically recognized syndromes and associations which were submitted (2,049) were accepted without any further follow-up. New or supplementary information was requested in the case of unspecified MCA (320). A copy of detailed necropsy records was requested from pathologists in lethal cases (2,022). Following these steps, apparent but not true instances of MCA were excluded (399), and an attempt was made to assign as many of the remainder as possible in 17 well-delineated MCA entities (900). The living index patients with severe MCA were referred where possible to the regional centers for evaluation (864). One hundred and seventy entities were identified, and seven cases were excluded as not representing MCA. In the socalled 3,393 unidentified cases for which no diagnosis was possible, the component abnormalities were tabulated according to their number. The final count was 6,643 cases with MCA, which is equivalent to a birth prevalence of 4.0 per 1,OOO total births, and to 10% of recorded cases with congenital anomalies. As a result of this program the proportion of recognized syndromes and associations among children with MCA increased from 29 % to 47 % . The accuracy of diagnoses has improved, e.g., the occurrence of unspecified cases decreased from 4.5% to 2%.