The aim of this study was to determine the efficacy of long-term maternal hyperoxygenation in comparison with bed rest in the management of mildly growth-retarded fetuses (5-10th centile). Thirty-eight patients with intrauterine growth retardation were studied. The patients were divided into oxygen-treated (n = 18) and untreated (n = 20) groups. Ultrasound assessment of amniotic fluid volume was performed on alternate clays, and the fetal abdominal circumference was evaluated weekly. Doppler analysis of fetal/maternal circulation was performed upon the patient's arrival at hospital, after 12 h, and thereafter on alternate days until parturition. Fetal blood was sampled by cordocentesis for immediate blood gas analysis at the start of the study and on the clay of delivery. Although in the treated patients there was significant improvement in Doppler flow pattern and blood gas indices, no significant differences in type of delivery and perinatal outcome were noted between the groups. Maternal hyperoxygenation in mild intrauterine growth retardation did not improve perinatal outcome.