A 6-week, double-blind, randomized, multicentre, parallel-group study was conducted to compare the efficacy of quetiapine ('Seroquel') (n=101) with that of chlorpromazine (n=100) in hospitalized patients with acute exacerbation of subchronic or chronic schizophrenia, or schizophreniform disorder. The tolerabilities of the two treatments were also compared. The mean daily doses of quetiapine and chlorpromazine at the end of the study were 407 mg and 384 mg, respectively. Both treatments were effective in the treatment of positive and negative symptoms, with a trend towards superior efficacy for quetiapine. The quetiapine group had a lower incidence of adverse events than the chlorpromazine group, and a low incidence of treatment-emergent extrapyramidal symptoms. Quetiapine was not associated with a sustained increase in serum prolactin. These clinical data support the preclinical profile of quetiapine as an atypical antipsychotic agent.