The International 10-20 system for EEG electrode placement is increasingly applied for the positioning of transcranial magnetic stimulation (TMS) in cognitive neuroscience and in psychiatric treatment studies. The crucial issue in TMS studies remains the reliable positioning of the coil above the skull for targeting a desired cortex region. In order to asses the precision of the 10-20 system for this purpose, we tested its projections onto the underlying cortex by using neuronavigation. In 21 subjects, the 10-20 positions F3, F4, T3, TP3, and P3, as determined by a 10-20 positioning cap, were targeted stereotactically. The corresponding individual anatomical sites were identified in the Talairach atlas. The main targeted regions were: for F3 Brodmann areas (BA) 8/9 within the dorsolateral prefrontal cortex, for T3 BA 22/42 on the superior temporal gyrus, for TP3 BA 40/39 in thearea of the supramarginal and angular gyrus, and for P3 BA 7/40 on the inferior parietal lobe. However, in about 10% of the measurements adjacent and possibly functionally distinct BAs were reached. The ranges were mainly below 20 mm. Using the 10-20 system for TMS positioning is applicable at low cost and may reach desired cortex regions reliably on a larger scale level. For finer grained positioning, possible interindividual differences, and therefore the application of neuroimaging based methods, are to be considered.