Polysomnographic examinations (PSG) were performed on 6 male schizophrenic outpatients who were being treated with benzodiazepine (BZD) hypnotics in combination with neuroleptics and 6 healthy male volunteers. In schizophrenic subjects, zopiclone (ZPC), 15 mg/day, was substituted for the BZD hypnotics, and PSGs were recorded again during ZPC therapy. All-night sleep stage scoring was carried out by visual analysis, and computerized period-amplitude analysis of sleep EEG was also performed. The schizophrenics showed marked reduction in the amount of slow-wave sleep (SWS) and in the number of delta half-waves during all-night sleep, especially those with higher amplitude, as compared to the normals. The number of delta half-waves in the patients was markedly reduced during the first sleep cycle. The average amplitude of delta half-waves during all-night sleep in the schizophrenics was significantly lower than that in the normals. The half-wave count of total delta waves in the schizophrenics was higher during treatment with ZPC than with BZDs, although no significant differences were observed in the amount of SWS between the two treatments. Soundness of sleep in the subjective sleep assessment was better evaluated during treatment with ZPC than BZDs. These results suggest that reduction of SWS in schizophrenia may be attributable mainly to the decrease in the number of delta waves with higher amplitude and that ZPC may induce deeper sleep in schizophrenics than BZDs.