Major depressive disorder (MDD) is a mood disorder that is often accompanied by the impairment of cognitive functions. The aim of this study was to investigate the feasibility of N270 as an index for evaluating the cognitive impairment in MDD patients. Twenty-five patients with MDD diagnosed according to DSM-IV and 25 age matched normal controls performed a matching task while event-related potentials (ERPs) were recorded from their scalp. There were two kinds of stimulus pairs in this study: match condition, the second stimulus (S2) in a pair was identical to the first one (S1); mismatch condition, S2 conflicted with S1 in the color attribute. Subjects were required to press a button in the match condition and to press another button in the mismatch condition. A negative ERP component, N270, which was considered to reflect conflict processing activity in human brain, was evoked by S2 of the mismatch condition. The patient group showed a delayed and smaller N270 than the control group. The prolongation of its peak latency was significant at P3 and P4 electrodes, and the reduction of its peak amplitude was significant at F3, F4, P3 and P4 electrodes. The amplitude of P300 elicited in the match condition was decreased in the patient group at P3 and P4 electrodes, but its latency did not differ from the control group. The results indicate that MDD patients as a group showed cognitive decline. N270 is a sensitive index in revealing cognitive impairment.