In eight anencephalic fetuses ultrasound observations of movement patterns were made and correlated with the morphological findings at postmortem. In all fetuses the movements were qualitatively abnormal: they were forceful, jerky in character and of large amplitude. In some of the most defective cases classification of the movements was hardly possible as they showed little similarity to those observed in normal fetuses. In these cases movements tended to occur in burst-pause patterns in contrast to being scattered over the record. Excessive activity occurred also only in the more defective cases. In fetuses with no evident cervical cord present isolated arm movements were observed. Fetal lung hypoplasia occurred as early as 16 weeks and in a fetus which showed both hiccups and breathing movements. It is concluded that with a severely defective fetal central nervous system, already in the first half of pregnancy movement patterns are abnormal. This abnormality mainly concerns the quality of the different movements. Secondly, movements can occur despite severe reduction in the amount and alteration in the organisation of the fetal central nervous system.