Our purpose was to develop a method of measuring the size of the brain stem by routine MRI and to determine brain stem dimensions in a normal population. We examined 174 subjects, aged 4 months to 86 years, with no known brain disease. Sagittal midline diameters of the mesencephalon, pons and medulla oblongata were measured on sagittal T1-weighted images, coronal diameters from axial T2-weighted images. The adult midsagittal diameter of the mesencephalon was reached at the age of 6 years, and decreased slightly after 45-50 years. Pontine dimensions increased until the age of 20 years and did not subsequently decrease. The midsagittal and midcoronal diameters of the medulla oblongata stopped increasing at the ages of 6 and 8 years, respectively. Minimal reduction in the midsagittal diameter occurs after 50 years. Normal ranges for each dimension were recorded. Knowledge of the normal variation in size of the brain stem can be helpful in the investigation of neurodegenerative diseases. The method described is rapid and needs no additional hard- or software. An additional finding was an increase in large vermian sulci in subjects over 50 years of age.