Abstract Most studies on event-related potentials (ERP) in psychiatric illness or dementia have focused on the single-peak latency of ERP components. In the present study, not only peak latencies of ERP components (N1, P2, N2, and P3) but also interpeak latencies (IPL; N1–P2, P2–N2, and N2–P3) were analyzed using the auditory oddball task. Thirty-five senile depressed patients and 34 patients with dementia of the Alzheimer type (DAT) were compared to 39 age-matched healthy volunteers. The mean latencies of P2 and P3 were shorter in patients with senile depression than in controls. In DAT patients, the N2 and P3 latencies were longer. When the IPL was assessed, however, only the mean IPL of N1–P2 was shorter in patients with senile depression, while the P2–N2 IPL were longer in those with DAT. The IPL results suggest that in senile depression the early cognitive process is hastened and in DAT the middle process is disturbed. Based on these results, we conclude that IPL of the auditory ERP might be used to reveal the disturbed steps within the cognitive process.