Haloperidol levels in blood were measured monthly in 43 refractory chronic schizophrenic patients referred to a locked skilled nursing facility for long-term treatment. Gross toxic side effects (seizures, catatonia, confusion) and Neuroleptic Induced Deficit Syndrome in conjunction with blood levels over 30 ng/ml were identified in 13 of our 43 patients. Blood level reductions contributed to a reduction of side effects and clinical improvement and led to the expedited discharge of 6 of these 13 patients of the toxic subgroup. Considerable blood level variation was evident in single samples, and four levels appeared necessary to develop confidence for accuracy. Significant dose to blood level interindividual variability was identified, thereby bringing into question fixed-dose approaches to patients. The results strongly suggest the utility of haloperidol blood levels in the clinical setting. (J Clin Psychopharmacol 1995;15:334-340).