Measurement of 6-ketoprostaglandin F1-alpha by radioimmunoassay was made during the first three days of life in a group of 48 preterm neonates at risk of intraventricular haemorrhage. The babies who developed haemorrhage had significantly higher levels than those who did not, and failed to show the falling levels seen over the first three days of life in the nonhaemorrhage group. It is suggested that high levels of prostacyclin in low birthweight babies may be one factor which contributes to the alterations of cerebral blood flow and capillary bleeding time which predispose to intraventricular haemorrhage.