Schizophrenics usually show smaller P300 waves than healthy subjects. It is uncertain whether this results from smaller P300 waves after single target stimuli or whether schizophrenics generate fewer P300 waves after target stimuli. To examine this, we elicited P300 waves in 10 healthy subjects and 17 schizophrenics in competitive visual stimulation with a checkerboard reversal pattern, using a correlation procedure to analyse single sweeps. We used two different patterns as reference: (i) a normal potential derived from the healthy subjects and (ii) the individual ERP as an individual reference. As a result we found that both healthy subjects and schizophrenics generate a wide variation of ERP responses subsequent to target stimuli and to non-target stimuli. Schizophrenics generate fewer ERPs after target stimuli (N-ref.: 40% vs. 60%) but more after non-target stimuli (N-ref.: 30% vs. 20%) in comparison to healthy subjects. This results in smaller P300 waves after target stimuli than in healthy subjects, but in larger P300 waves subsequent to non-target stimuli relative to healthy subjects. Thus, the single-sweep analysis demonstrates an overlap of the processing of target and non-target stimuli in schizophrenics. This suggests a decreased ability of schizophrenics to maintain a focus of attention, resulting in a decreased ability to differentiate between relevant and irrelevant stimuli.