Damage to the optic radiation during surgery in the temporal lobe results in more or less severe visual field defects. In the present study, the optic radiation was represented by either Klingler's fibre dissection technique or by consecutive frontal sections respectively in 25 formalin-fixed human hemispheres. This enabled one to gain exact data on the three-dimensional course, the length and other measurements of this important fibre system. In addition, its relations to the temporal horn, the trigonum and the occipital horn of the lateral ventricle were determined. The results of these measurements are discussed with respect to various surgical interventions involving the temporal lobe.