Magnetic resonance imaging is a simple method for evaluating the tectum. Until now there was no satisfactory method that could detect small lesions of the tectum area in living humans. In order to determine an abnormal increase in size of the tectum, we gathered data about the normal tectum. Measurements on mesencephalic tectum were done in 100 korean adults by midline sagittal T1-weighted image using the spin echo technique with a 2.0 Tesla MR unit. The mean diameters of the superior colliculus(A), intercollicular groove(B), and inferior colliculus(C) were 3.0, 2.3, and 3.0mm, respectively, and the mean length of the tectal plate(D) was 12.7mm. There was statistically significant thickening of A, B, and C in conclusion the measurement of the mesencephalic tectum using magnetic resonance imaging is useful for the evaluation of small infiltrating lesions of the tectum itself, as well as Chiari ll malformation or aqueductal stenosis by periaqueductal lesions.