Objective: To study the cognitive functions of rehabilitated hearing impaired (HI) patients using auditory perception skills and P300 event related potential. Methods: Aided speech discrimination score (WDS %) and acoustic P300 were recorded in free field for 40 rehabilitated (hearing aids, cochlear implants) HI adults and compared to 10 normal hearing controls. Results: Rehabilitated HI patients had statistically significant longer P300 latencies and less WDS % compared to the controls. There was no statistically significant difference between mean P300 latency without and with the device at any degree of hearing loss. There was a statistically significant inverse correlation between P300 latency and age of hearing loss and WDS%, and a direct correlation between P300 latency and duration of hearing loss and hearing threshold. There was a statistically significant difference as regards regular use of the device among subjects with different speech performance categories. Subjects who use the device regularly showed higher percentage of positive responses than subjects who don’t use it regularly. Subjects with excellent speech performance showed the statistically significant lowest mean P300 latencies. Conclusion: Amplification increases the detectability of P300 and shortens its latency. P300 elicited by tonal contrasts is a neurophysiologic indicator of the discrimination abilities.