The Talairach stereotactic proportional grid method for telencephalic localization was applied to magnetic resonance imaging of 30 brain hemispheres of healthy adult volunteers. Using sagittal images, outlines of the following characteristic principal sulci were identified on the brain surface to validate the method: callosal sulcus, parietooccipital sulcus, marginal sulcus, superomedial portion of central and postcentral sulcus, and posterior sylvian fissure. The maximal variation zones of sulcus location obtained by superimposition of the individual grid data on a standard proportional grid were similar to those originally reported by Talairach from pneumoencephalographic studies. However, the central sulcus, which was only studied postmortem by Talairach, was located 0.5-1 cm more posteriorly in the present study. Magnetic resonance also provided more detailed data on the considerable variation of the terminal parts of the sylvian fissure and on the well-known left-right asymmetries. The left sylvian fissure extended farther posteriorly in 11 of 15 brains. With the exception of this posterior perisylvian region, the proportional grid proved to be valid for indirect cerebral cortical localization in the regions studied.