Head ultrasound examinations of 102 infants were reviewed for the presence of a cavum septi pellucidi (CSP). The CSP was seen in 42% of the entire population, 61% of premature infants, and 50% of full-term infants. The largest CSP measured 10 mm in width. The ultrasound image of the CSP is presented to familiarize radiologists with this normal structure, and the distinction between a dilated ventricular system and the CSP when imaging the fetus with ultrasound is discussed.