The ultrasonographic findings and neurological development of 15 preterm infants, born between August 1981 and July 1984, who developed periventricular leukomalacia (P.V.L.) are described. The incidence of P.V.L. in infants with a birth weight less than 1 500 g was 2.3%. Nine of the 15 infants demonstrated areas of increased echogenicity in the periventricular regions prior to the development of cystic changes. The mean age for the appearance of cysts was 26 days. At follow-up 14 infants showed evidence of abnormal motor development and 8 infants had visual defects. All infants appear to have normal hearing, but 3 infants show signs of delayed speech. To date, only 2 infants demonstrate signs of mental retardation, but many of the infants are too young to assess intelligence accurately.