Plasma haloperidol levels were compared to clinical response during the first 14 days of drug treatment in 14 DSM III-diagnosed inpatient schizophrenic patients using an improved methodology, which utilized predetermined constant dosages and derived a therapeutic range of plasma haloperidol levels from a curvilinear regression analysis. An inverted U-shaped relationship was found which reflected a significant curvilinear correlation (r = 0.66, P less than 0.02) between plasma levels (as assayed by gas chromatography) and improvement on the Serial New Haven Schizophrenia Index. A therapeutic window was suggested by the present study, with optimum patient response associated with plasma haloperidol levels of 4.2-11.0 ng/ml for the first 2 weeks of treatment.