This study has attempted to assess the burden imposed by congenital anomalies in terms of postnatal mortality and morbidity, which were in turn used to classify anomalies as severe and mild types. Factors studied were postnatal mortality through age 7 years and morbidity, as measured by neurologic and psychologic abnormalities, histories of major surgery, prolonged hospitalization, and chronic infections. The study was based on a prospective study of 52,332 liveborn singletons of the Collaborative Perinatal Project of the National Institute of Neurological and Communicative Disorders and Stroke. In general, the highest degree of burden was observed in syndromes and sequences, followed by multiple and single major anomalies. The burden due to major abnormalities as measured by attributable risk ranged from 0.436 for prolonged hospitalization up to one year, to 0.010 for chronic infections in subjects 1-7 years of age. In terms of mortality, the total attributable risk was 0.164, and the mean potential years of life lost was 5,020 per 10,000 population, which is considerably greater than that reported in other studies. An index constructed from mortality, neurologic, psychologic, and surgical variables provides a reasonable and objective means for classifying anomalies into severe and mild types.