The aim of this study was to quantify the intra- and extraventricular cerebrospinal fluid (CSF) spaces in children with benign enlargement of the frontal subarachnoid space (BE). The infra- and supratentorial CSF compartments were measured in 61 CT examinations of children with BE, 3-27 months old, and compared with those of 96 CT examinations considered normal. Measurements of the ventricular system, and the pontine and chiasmatic cisterns were related to cranial size. In all children with BE the lateral and third ventricles were dilated and the chiasmatic cistern was widened. The subarachnoid space was wider than the upper limits in the control group, in the frontal region (4 mm), and the anterior interhemispheric (4 mm) and Sylvian (3 mm) fissures. The infratentorial CSF compartments, the occipital subarachnoid space, the posterior part of the interhemispheric fissure and, in most cases, the cortical sulci were normal in size in children with BE. The majority were macrocephalic or had rapid head growth but there were also normocephalic children with normal head growth. The size of the posterior fossa was within the normal range in all children with BE. Idiopathic BE is not uncommon in children up to about 3 years old who are healthy or have minimal neurological disturbance and is characterised by a specific pattern of widening of the supratentorial CSF compartments.