PURPOSE: To determine the clinical significance of prominent extra-axial CSF space (EACSFS) in infants, as seen on cranial ultrasound. MATERIALS AND METHODS: Between March 1996 and November 1997, all infants who had undergone head ultrasound at our institution and were found to have prominent EACSFS were evaluated. The width of the interhemispheric fissure was measured at three locations at the level of the frontal horn, body and atrium of the lateral ventricles. The depth of the CSF space over the convexity was also measured. The average of these measurements was calculated and each patient was assigned to one of three groups: mild, moderate, or marke d. Ultrasound findings were evaluated for other associated abnormalities. Clinical neurodevelopment was evaluated by a pediatric neurologist, and ultrasound and neurodevelopmental findings were correlated. RESULTS: Prominent EAC S FS was found in 153 patients, and neurodevelopmental evaluation up to a corrected age of 9 months was available in 133. One hundred and eight of 117 infants with normal neurodevelopment had no other associated abnormality (n=81), or abnormality associated only with grade I subependymal hemorrhage or cyst (n=27). Twe l ve of 16 infants with an abnormal neurodevelopmental outcome had major abnormalities including PVL, grade IV hemorrhage, and marked ventriculomegaly. CONCLUSION: Prominent EAC S FS alone does not appear to be clinically significant. An abnormal neurodevelopmental outcome is associated with major abnormalities seen on ultrasound. Follow-up examination for prominent EAC S FS is not indicated unless the associated abnormality requires further evaluation.