The lung surfactant phospholipid composition of lavage samples from 102 babies dying from Sudden Infant Death Syndrome (SIDS) (one-third with minor signs of inflammation) was compared with that of: 34 babies dying from Hyaline Membrane Disease (HMD), 15 mature babies dying soon after birth, 16 mature babies dying in the same age range as the sudden infant death syndrome cases, 13 babies dying from pneumonia and 6 from septicaemia. The surfactant of the two groups of babies dying from SIDS was identical and approximated that obtained from babies dying from HMD, pneumonia or septicaemia. Compared to that obtained from mature babies, the surfactant of babies dying from SIDS contained significantly lower proportions of phosphatidylcholine (PC) and significantly higher proportions of lyso-PC and sphingomyelin. The proportion of disaturated PC was similar to that of the surfactant of the age-matched mature babies. The surfactant composition of the babies dying from SIDS did not change appreciably after death nor vary with age at death. The surfactant phospholipid composition of postmortem samples from mature babies was similar to that of aspirates from living babies and infants and to that of bronchoalveolar lavage samples from living adults.