The aim of this study is to clarify the timing of brain injuries in preterm infants who later developed West syndrome. We assessed the timing of brain insults with serial EEG recordings beginning immediately after birth in preterm infants less than 33 weeks of gestational age. The timing of brain insult was assessed by considering EEG findings in relation to the time of birth. The timing was considered prenatal if an EEG immediately after birth already showed chronic stage abnormalities, perinatal if it showed acute stage abnormalities, and postnatal if it was normal and a later EEG showed acute stage abnormalities. Eleven infants had periventricular leukomalacia and developed West syndrome. The timing of the brain insult was considered prenatal in 5 infants, perinatal in 2, postnatal in 2, prenatal plus postnatal in one, perinatal plus postnatal in one. The infants of less than 29 weeks of gestation were more likely to suffer brain damage postnatally, whereas those of more than 28 weeks tended to have brain injury prenatally. The timing of brain injury of preterm infants who later developed West syndrome is more often prenatal than generally considered.