The authors investigated the clozapine plasma levels of 148 psychiatric inpatients. Multiple regression analysis revealed a linear relationship between dose of clozapine and plasma concentrations. The analysis showed a significant influence of dose, sex, smoking, weight, and age on the plasma concentrations of clozapine under clinical conditions. These results remained significant when clozapine doses below 150 mg/day and above 500 mg/day were excluded from the analysis.