The subarachnoid space was examined with real-time ultrasonography (US) in 89 healthy infants. US of the brain in all infants revealed no abnormalities. Three variables were measured in the coronal plane at the level of the foramen of Monro: the sinocortical width (SCW) ranged from 0.4 to 3.3 mm, the craniocortical width (CCW) from 0.3 to 6.3 mm, and the interhemispheric width (IHW) from 0.5 to 8.2 mm. All variables can be used routinely, as the SCW could be demonstrated in all infants, and the CCW and IHW were demonstrated in 96% (85 of 89). Correlation of sonographic measurements with the independent variables age, head circumference, body weight, and body length was poor. To differentiate normal from pathologically dilated subarachnoid spaces, the following upper limits are proposed on the basis of the 95th percentile: 3 mm for SCW, 4 mm for CCW, and 6 mm for IHW.