This case describes the prenatal diagnosis at 30 weeks' gestation of a posterior fossa subdural hematoma. Doppler velocimetry studies of the middle cerebral artery at that time showed an abnormally high resistance pattern with reverse end-diastolic flow. Ultrasonic assessment of the fetus indicated that there was associated quadriplegia. No cause for the lesion was demonstrated. A fetal blood sample, obtained by cordocentesis, revealed a normal karyotype. Fetal blood gas analysis was also normal. The pregnancy was terminated at the parents' request.