We compared three chromatographic methods for the therapeutic control of clonazepam. The first method analyzes the underivatized benzodiazepine, direct gas chromatography (GC) method, using nitrazepam as internal standard and GC with an electron-capture detector (63Ni). This method was compared with a second one, in which the same compounds were converted to the respective methylated derivatives (GC of derivatized samples) and also quantified by GC. The two methods were then compared with a third procedure for clonazepam quantification, using high-performance liquid chromatography (HPLC). Statistical analysis of clonazepam concentrations detected in the plasma of 37 patients, treated with Rivotril, indicated that HPLC is the most precise method, showed just significant correlations among the three methods, and suggested that plasma concentrations of clonazepam, estimated by the direct GC method, are significantly lower than those determined by the other two methods. The direct GC method and HPLC have been proposed as reference methods for the therapeutic control of clonazepam, with options for use left to the resources available in various laboratories.