Assessments of prepulse inhibition and habituation of the acoustic startle response have proved to be valuable tools for assessing deficits of sensorimotor gating and information processing in schizophrenia patients. Recent studies, however, have reported inconsistent results regarding startle habituation deficits in schizophrenia using block-to-block analyses. Some of these inconsistencies may be due to abnormal initial sensitization effects to startle-eliciting stimuli. In a longitudinal study during the course of an acute psychotic episode, 34 medicated inpatients were examined with regard to sensitization and habituation effects in a trial-by-trial analysis and compared with 18 normal control subjects. On two examinations--10 days after admission and after psychopathological improvement 2-3 weeks later--schizophrenia patients exhibited an exaggerated magnitude increment across the first few startle-eliciting stimuli and habituation deficits that were evident when the effect of sensitization was removed from analysis. In the present study, both increased sensitization and reduced habituation appeared to be trait markers of schizophrenic psychoses. The enhanced sensitization effect--presumably due to an abnormal arousal modulation--reflects abnormal stimulus processing in schizophrenia, i.e. the diminished ability to learn the irrelevance of simple identical stimuli. In addition, the present data have important implications for designing startle studies to assess sensitization, habituation and prepulse inhibition in one session.