To examine the left temporal scalp area reductions of P300 amplitude, event-related potentials (ERPs) during a standard oddball task were recorded in 57 schizophrenic patients and 33 normal controls. The P300 amplitude at T3 was not significantly smaller than that at T4 in schizophrenic patients. In the ANOVA analysis of the P300 peak amplitude and PCA factor scores, significant lateral topographical differences in P300 were not present between patients and controls. However, in schizophrenia, patients in the low T3 P300 group were older and consuming higher doses of antipsychotic medicine than those in the high T3 P300 group, and they had relatively low P300 amplitudes and significantly delayed P300 latency, compared with those in the high T3 P300 group. These findings suggested that although the reduction in the left temporal P300 amplitude did not necessarily exist in schizophrenic patients, it may be associated with the severity of the disease process and/or impairment of cognitive function.