Intra- and extraventricular subarachnoid spaces in children were studied by high-resolution computed tomography. Scans were reviewed of 34 patients who were selected as highly likely to have normal scans. Sizes of the ventricular system and the seven extraventricular subarachnoid compartments were analyzed and graded on a subjective scale from 0 (not visible) to 4 (markedly enlarged). Data were also analyzed by age group (greater or less than 2 years of age). The subarachnoid spaces were found to be both larger and more variable in size before the age of 2 years and to be quite uniform thereafter. Based on these findings, it is inadvisable to base specific diagnoses during the first 2 years of life solely upon modest enlargement of the subarachnoid spaces.