Fetal breathing movements (FBM) were observed daily using a real-time B-mode ultrasound method in a patient with sickle cell anemia in crisis. Observations were made on 2 occasions in the presence of maternal hypoxemia (PO2 ≤ 40 mmHg), and FBM were noted to be absent. Conversely, when maternal Po2 was 60 mmHg or greater, FBM were present 23–80% of the time. The FBM were reduced or absent within 90 minutes of maternal Demerol injection. These observations suggest that the human fetal response to hypoxemia may be similar to that observed under experimental conditions in the animal fetus.