The authors treated 16 outpatients and 1 inpatient who had diagnoses of schizophrenia in exacerbation with haloperidol as the sole neuroleptic agent and obtained ratings of psychopathology and serum levels of haloperidol. Improvement in schizophrenic symptoms measured by the Brief Psychiatric Rating Scale was significantly greater in patients who had mean haloperidol serum concentrations in the range of 8-18 ng/ml than in patients whose mean haloperidol serum concentration fell outside this range.