The authors report the natural history of closure of the cavum Septi pellucidi in premature infants 26 to 27 weeks postconception at birth and compare the developmental outcome in these infants who had closure by 42 weeks postconception to those who still had a cavum septum pellucidi visualized on ultrasound at approximately term (35-42 weeks). Of 72 patients, 35 patients still had a cavum septum pellucidi visualized on the last ultrasound done between 35 and 42 weeks postconception, and the developmental outcome of these patients was no different from those with earlier closure. The authors conclude that persistence of a cavum septi pellucidi through term is not an independent risk factor for developmental delay.