The aims of this study were to determine the types of chronic-stage EEG abnormalities that exist and to clarify their relation to neurodevelopmental outcome in preterm infants. We evaluated 183 preterm infants with gestational ages of less than 33 weeks (mean age 29.2 weeks) and weighing less than 2000 g (mean weight 1275 g). The first EEG was performed within 72 hours of life; thereafter, EEG was performed once every 1 to 4 weeks until the infant reached a post-conceptional age of 40 to 42 weeks. Two kinds of EEG abnormalities, acute- and chronic-stage abnormalities, were evaluated and we assessed mainly the latter. Chronic-stage EEG abnormalities were divided into two patterns: disorganized and dysmature. Periventricular leukomalacia (PVL) and intraventricular haemorrhage (IVH) were diagnosed on the basis of ultrasound findings. Psychomotor development was examined every 3 months after discharge until at least 18 months of the infants' corrected age. Disorganized and dysmature patterns were observed in 52 and 28 infants respectively. Among the 52 infants with disorganized patterns, PVL was observed in 31 and IVH in seven infants. Thirty-nine infants had cerebral palsy (CP). Twenty-six achieved normal cognitive development. Of the 28 infants with dysmature patterns, PVL was seen in one and IVH in 11 infants. CP was seen in five infants. Only eight infants achieved normal cognitive development. Gestational age and birthweight were significantly lower in infants with dysmature patterns than in those with disorganized ones. Results indicate that types of chronic-stage EEG abnormalities are related to types of neurological sequelae and are useful for assessing the mode of brain injury in preterm infants.