The lipid composition of lung surfactant obtained by lung lavage at autopsy in 40 infants dying from the sudden infant death syndrome (SIDS), was compared to that obtained from 12 infants dying from other causes (control group). Analysis of the lipids from the two groups showed no major difference in the proportions of the various phospholipid classes particularly the predominant component, phosphatidylcholine (PC), which was present at 60.7 +/- 0.9% (mean +/- S.E.) of the total phospholipids in the SIDS group and 57.9 +/- 2.9% in the control group. However the proportion of the PC present as the disaturated form (DSPC), was significantly (P less than 0.01) reduced in the SIDS group (65.8 +/- 1.6%) in comparison to the control group (77.4 +/- 3.5%). The proportion of DSPC present in the PC fraction of SIDS infants in the high-risk age range of 1-26 weeks (63.9 +/- 1.9%) was also significantly reduced (P less than 0.01) in comparison to the total control group of infants. For infants older than 26 weeks, significant differences in the proportion of DSPC in PC were not observed between SIDS and control groups. A functional consequence of the observed reduction in the DSPC content of lung surfactant of SIDS infants could be a greater degree of fluidity of the surfactant, particularly at exhalation. Such a biophysical change in surfactant properties could have a profound influence on lung function and be a causative factor in sudden infant death.