Introduction: Mismatch negativity (MMN), in the deviant-minus-standard event-related potential (ERP) difference-waveform, may represent a working memory trace for tone differences (a deviant among a sequence of standards). Most, but not all studies find MMN reduced in patients with schizophrenia. Aims: This report investigates if differences may be attributable to experimental condition (diffuse vs focused attention), component identification (N1-like vs N2-like), topographic distribution and clinical condition (with / without paranoid -hallucinatory symptoms, PH/NP). Comparisons were made for 12 PH and 12 NP schizophrenic patients with 13 obsessive compulsive and 25 normal control subjects. Results: Frontal MMN reduction in schizophrenics largely resulted from an absence of an increase in focused attention conditions as in comparison groups. But there was marked activity recorded from sites over the temporal lobe in NP patients. These features were not reflected in other components except for a visible but nonsignifiant N1-like temporal locus in NP patients. Further, schizophrenic patients did not show an increase in late positivity with focused attention like the comparison groups. Conclusions: The results show that so-called automatic processing deficits in schizophrenia (amount and locus of MMN) are best seen in situations requiring the activation of controlled attentional processes. It is suggested that impaired processing of irrelevant stimuli and reduced frontal MMN in NP patients may reflect reduced dopaminergic responsivity.