Objective. Amplitude and latency of evoked potential P300 are considered as major correlates of cognitive functions. At the same time the changes in the theta-band event-related oscillations that appears at the same time window as P300 wave are less studied. The purpose of this study was to provide the complex assessment and comparison of neurophysiological indices of cognitive processes in healthy people and patients with personality disorder, schizotypal disorder and schizophrenia with different levels of cognitive dysfunction. Material and methods. The total number of participants was 124, including 44 healthy volunteers, 40 schizophrenic patients, 22 patients with personality disorder, 18 patients with schizotypal disorder. Patients were examined in standard two-stimulus oddball paradigm. The P300 amplitude and latency, and evoked theta-band power and coherence were identified for target and non-target stimuli. Results and conclusion. The decrease in P300 amplitude, prolongation of P300 latency, and reduction in the evoked theta-band power and coherence were revealed in all groups of patients compared to healthy subjects. These changes were spatially generalized and the most pronounced in schizophrenic patients. In patients with personality and schizotypal disorders, the changes had a local character, and didn’t differ significantly when the groups were compared. The results show the gradual cognitive decline as follows: healthy patients, patients with schizotypal disorder, patients with personality disorder, and patients with schizophrenia. Such cognitive decline might be in line with the reduction of critical and prognostic abilities.