Amplitude reduction of the oddball P3 wave is a well-replicated but non-specific finding of schizophrenia. The time-frequency analysis of single-trial ERP data allows to specify in a reliable manner whether the P3 reduction in schizophrenia is due to the decreased P3 response in single trials or due to the inter-trial variability in the timing of the response. Since the delta response most strongly contributes to the P3 amplitude, we focused to the low frequency range of the time-frequency transformed data. EEG was recorded from chronic schizophrenia patients and matched healthy controls during a simple visual oddball task. The wavelet transforms of the averaged ERP and the single trials were computed to investigate the amplitudes of the evoked (phase-locked) and total (phase-locked+non-phase-locked) delta (1-3 Hz) responses, respectively. Evoked delta activity and P3 amplitude to target stimuli were both reduced significantly in patients with schizophrenia, whereas no such difference was obtained for the total delta activity. The significant reduction of the evoked delta response and the absence of such a difference in the total delta response of schizophrenia patients reveals that the delta band response is weakly phase-locked to stimulus in schizophrenia. This result suggests that the reduced P3 amplitudes in the averaged ERPs of schizophrenia patients result from a temporal jitter in the activation of neural circuits engaged in P3 generation.