INTRODUCTION: Schizophrenia is a chronic and disabling psychotic disorder associated with cognitive impairment which affects the patient’s functional outcome. P3a is an event-related potential which seems to be involved in automatic orienting and involuntary attention processes. Reduced amplitude of p3a has been consistently observed in schizophrenia patients. However, most studies have reported p3a deficits using passive oddball paradigms in medicated patients (Atkinson, 2012; Jahshan, 2012; Kaur, 2011; Nagai, 2013; Takahashi, 2013). To our knowledge, no reports of p3a elicited using an active task in unmedicated patients have been performed. Thus, the aim of the study was to analyze the p3a component in antipsychotic-naïve first-episode psychosis patients (FEP) and healthy controls (HC), using an active oddball paradigm. METHODS: 15 FEP patients and 13 HC were recruited for p3a recording at the National Institute of Neurology and Neurosurgery in Mexico City. All participants signed an informed consent letter. Subjects performed an active auditory involuntary attention task including frequent (1000 Hz, 90%) and deviant (900 and 1100 Hz, 10%) tones during an EEG recording. They were instructed to classify the tones according to their duration (i.e., long vs. short) regardless the pitch. RESULTS: Mixed ANOVA analyses showed that p3a amplitude was significantly reduced in antipsychotic-naïve FEP patients when compared to HC. The main effect was observed at frontocentral electrodes (p < .01). No differences between groups were found regarding p3a latency. CONCLUSIONS: Decreased p3a amplitudes are evident in antipsychotic-naïve FEP patients. Such deficits indicate that involuntary attention processes may be affected at early stages of the disease. Moreover, such impairments are independent from medication. Thus, p3a amplitude reduction may be a core feature of schizophrenia and should be considered as a psychophysiological trait marker of the disease.