The studies of relationship between haloperidol plasma levels and clinical response in schizophrenia have yielded variable results. A therapeutic window for haloperidol may exist, but the evidence for it is inconsistent. Future studies of this problem using larger patient samples would permit the investigation of interactions between the clinical predictors and the haloperidol plasma levels in the determination of clinical outcome. Although the reduced haloperidol in plasma as well as the red blood cell levels of haloperidol and reduced haloperidol or ratios of the metabolite to parent compound may also be related to clinical outcome, the evidence for this relationship is very tentative. More research is needed before routine clinical monitoring of haloperidol blood levels can be recommended.