Three patients who were at risk of having a baby with a neural-tube defect were investigated by ultrasonic examination and alpha-fetoprotein (A;F.P.) estimation. In all three cases amniotic-fluid A.F.P. levels were significantly raised and pregnancy was terminated. In the first case ultrasound failed to detect a small lumbo-sacral meningocele; In the second case ultrasound successfully diagnosed a dorso-lumbar meningo-myelocele but did not recognise internal hydrocephalus. In the third case ultrasound did not define any lesion, and pathological examination of the fetus revealed no abnormality. It is concluded that ultrasonic examination is an important diagnostic technique in the antneatal investigation of spina bifida.