The aim of this open study was to determine a more rational therapeutic approach for psychotic patients treated with clozapine for several months, using measurement of plasma and red blood cell levels (P, RBC) of clozapine (cloza) and N-desmethylclozapine (descloza), the major metabolite of clozapine, which has been reported to be less active but more toxic (agranulocytosis) than clozapine itself. The RBC concentration may be considered as more representative of the free fraction drug. The study concerned 7 patients suffering from chronic paranoid schizophrenia according to the DSM-IV criteria. All of them were treatment-refractory schizophrenic inpatients (4 men, 3 women, mean age +/- SD: 38.2 +/- 8.4 years; mean duration of illness +/- SD: 14.4 +/- 5.1 years). They had received at least two different neuroleptics, for 6 weeks, before entering the study. Treatment started in our hospitalization unit with clozapine 25 mg up to a maximum of 900 mg/d (mean stabilized daily dose +/- SD: 507 +/- 211 mg and mean daily dose per kg: 6.91 +/- 3.08 mg). Clinical evaluations (Quality of Life Scale: QLS), regular blood monitoring and biological samples were conducted at the same time, weekly for 18 weeks and then monthly (duration of the study: 4 to 38 months; mean +/- SD: 12.9 +/- 11.5 months). Plasma and RBC (after lysis) levels were determined by reversed phase HPLC and UV detection after extraction with hexane. All the patients improved very quickly after the first week of treatment and six were able to leave the hospitalization unit and start outpatient care such as daily hospitalization, returning home or in sheltered accommodation. With the following plasma (P) and RBC levels: mean cloza +/- SD: (P = 294 +/- 146 ng/ml; RBC = 110 +/- 82 ng/ml) and mean descloza +/- SD: (P = 173 +/- 106 ng/ml; RBC = 76 +/- 54 ng/ml); none of the seven patients developed agranulocytosis. The blood levels, ensuring better surveillance, have a predictive value for clinical improvement. A linear pharmacoclinical correlation was only found between RBC cloza concentrations and the evolution of the QLS scores. Clozapine fulfils the criteria for therapeutic drug monitoring, and determination of plasma, and more particularly RBC, cloza and descloza levels may help to find the lowest effective dose with the fewest side effects.