BACKGROUND The speech recognition performances in postlingually deaf patients, which had a long duration of deafness, seem to be poorer than in patients with a short duration of deafness. The reason could be a functional reorganization in the auditory cortex. A long time of auditory deprivation may decrease neuronal activity in the auditory related cortices. As the late acoustically evoked potentials are generated in this region, we compared the speech recognition performances and also the late acoustically evoked potentials with the individual duration of deafness in a series of postlingually deaf patients. METHODS In 9 patients (cochlear implant (CI), MED EL) with postlingual deafness the late acoustically evoked potentials and the HSM-set-test on the quiet were measured. Additionally 1 patient with prelingual deafness was examined. The CI-operation was done, when he was 14 years old. The relations between the late acoustically evoked potentials and the speech recognition performances in dependence of the duration of deafness were examined. RESULTS Typical late acoustically evoked potentials could be measured in 3 patients only. The speech recognition in these patients was very good. The longest time of deafness in these patients was 6 years. In 6 patients the minimum duration of deafness was 12 years. Here were measured the N1-potentials only, the P2- and the N2-potentials were absent. In these patients the scores of the HSM-set-test were lower than in the others. In the 15-years old patient with prelingual deafness the speech recognition was absent. The late acoustically evoked potentials of this patient showed an atypical form. CONCLUSIONS The results of this study showed a correlation between the duration of deafness, the forms of the late acoustically evoked potentials and the speech perception. The duration of deafness preceding CI-implantation should be very small in deaf children and in postlingually deaf patients.