Relationships between abnormalities on neonatal serial cranial ultrasound and cognitive development at age one year were examined in 153 low birth weight (LBW) infants. Infants with complex injury (persistent parenchymal echogenicity, lucency, or persistent ventricular enlargement) scored significantly lower on the Bayley Mental Development Index than noninjured infants. Nine of 11 infants with complex injury had severe developmental delay in contrast to 3/110 of the noninjured. Adjusting for birth weight, gestational age, head circumference and social class, infants with complex injury were 33 times more likely to be severely delayed than noninjured infants. Risk for severe delay associated with LBW appeared to be indirect, through increased probability of ultrasonographic abnormality. The poorest developmental outcome was seen in infants with both complex perinatal brain injury and either very LBW or very young gestational age. However, very LBW infants with normal neonatal ultrasounds were at negligible risk for severe delay at age one.