The term "external hydrocephalus (EH)" is used to describe excessively rapid head growth in infants who are found to have enlarged subarachnoid spaces and little or no ventricular enlargement. EH has not yet been described in discordant birth weight twins. We report the cases of two sets of discordant twins with normal neonatal course and normal neonatal brain ultrasound scans who developed an EH in the first months after birth and had a mild neurodevelopmental delay. Our findings suggest that EH may be part of neurologic morbidity in discordant birth weight twins and represents one of the potential complications to look for in the follow-up of these infants.