Background and Purpose. Controlling for intracranial volume is crucial in magnetic resonance studies of changes in brain volumes, However, full intracranial volume measurement requires prolonged scanning and analysis, and simple, validated methods for intracranial volume estimation are lacking. The authors developed and evaluated a method of estimating intracranial volume using a single mid-sagittal slice, the intracranial cross-sectional area. Methods. Forty men aged 65-70 had whole skull magnetic resonance scans at 1.9 T, with slice thickness of 1.5 mm (no interslice gap). Intracranial cross-sectional area was traced in the midline sagittal slice of the 3-view localizer. Intracranial volume was measured using every slice. Ten intracranial cross-sectional area measurements were tested for interrater reliability. Results. Intracranial cross-sectional area and intracranial volume correlated highly (r = .88, P < .0001). A modified Bland-Altman plot showed good agreement between intracranial cross-sectional area and intracranial volume. The intraclass correlation (an indicator of reliability) for intracranial cross-sectional area was r = .976 (P < .005). Conclusions. The rapid and simple technique of intracranial cross-sectional area measurement provides a valid and reliable estimate of intracranial volume.