Ambient cistern (AC) is a thin extension of the subarachnoid space surrounding the brainstem at the level of the mesencephalon and pons. Despite various definitions, it constitutes an important landmark in clinical assessment of intracranial volume reserve. Although it is indisputably useful, there exists no defined standard for radiological examination for the dimensions and ranges in specific age groups. This paper aims to describe the ambient cistern anatomically and give the ranges of dimensions for proper radiological interpretation. The study was performed on 160 axial computed tomography (CT) examinations of Polish children of both sexes, aged 1-18 years, admitted to the hospital because of mild brain concussion. Pictures were made using a Siemens 8-row CT scanner, without contrast administration. We estimated distances at the level of the pons and midbrain, based on axial cross-sections, according to standard radiological protocol. The parameters included the width of the AC in its anterior and posterior part, the width of the tentorial notch, and the distance from the pons and sella. All measurements were analyzed statistically with StatSoft Statistica 8.0 software. The average width of the AC differs between age groups. It is greatest at 1-3 years (2.8 +/- 0.6 mm) and lowest at 4-10 years (2.4 +/- 0.6 mm). AC is more likely to be greater in its anterior part in boys. The distance from the sella to the pons is greatest in 1-3-year-old girls (6.9 +/- 1.3 mm), and the tentorial notch is widest in the 15-18-year-old group (24.6 +/- 2.4 mm). Dimensions of the AC correlate with intracranial reserve volume. This is particularly visible in the youngest children. Thin and narrow AC is not always a sign of raised intracranial pressure. It may be specific for the child's age.