Adapting Sir Dugald Baird's concept of primary obstetric causes of perinatal mortality, a revised clinico-pathological classification has been evolved to take account of new knowledge and developments, and to direct attention to potentially avoidable deaths and to where intensified efforts and investigation are needed. Categories highlighting the importance of intrauterine growth retardation, unexplained intrauterine death and spontaneous premature labour have been introduced, intrapartum hypoxia is separated from birth trauma, and infection again has its own category. Regular perinatal audit at one obstetric hospital, since 1979, has shown that the new system provides a workable and useful means for classifying not only perinatal deaths, but also late abortions, late neonatal deaths and perinatally related infant deaths. The rate of total perinatally related wastage, defined in this way, was almost twice that for perinatal mortality (22.8 compared with 11.9 per 1000 births). The former is advocated as a more realistic index for the audit of perinatal care. The revised and extended system is put forward as a contribution to the current debate on classifying and reporting such wastage, in the hope that it may be tested as a model for regional as well as hospital surveys.