Fetal oxygenation studies have provided the obstetrician with some basic information regarding oxygen supply to the fetus. Although standard values for normal fetuses at term have been made, little has been done to evaluate any changes occurring with fetal and neonatal abnormality. Twenty-two sets of analyses were performed to measure oxygen affinity (P50) 2,3-diphosphoglycerate concentration and full blood gas analysis on umbilical arterial and venous samples. The samples were taken from a random obstetric population. All babies were at term or post term by antenatal data and paediatric assessment at delivery. A further study was performed on a group of normal and growth retarded term infants, assessing the same parameters as the random population. Following analysis, 2 cases of cardio-pulmonary abnormality, resulting in neonatal death, were found to have the highest oxygen affinity, the remaining oxygenation parameters of PO2, PCO2, pH and 2,3-diphosphoglycerate concentration were found to be similar to the rest of the group. The results of this study agree with previous investigative findings for the normal babies. The finding of low P50 (or high oxygen affinity) in these 2 cases is an unusual finding and may reflect an adaptive function towards extra-uterine life. No alteration of fetal oxygen affinity was found in a small group of growth retarded babies despite the lowering of the 2,3-diphosphoglycerate concentration. The latter finding probably reflects the lowered intrauterine fetal blood pH found with growth retarded babies. Further research is required to elucidate the comples process of fetal oxygenation that occurs in the presence of fetal abnormality.