The authors report preliminary findings on the potential contribution of CYP2C19 isoenzyme to the human metabolism of N-desmethylclobazam (N-CLB), the main active metabolite of clobazam (CLB), a benzodiazepine frequently used as add-on therapy in patients with refractory epilepsy. Two children on CLB treatment showing extremely high plasma concentration/dose ratio (C/D) of N-CLB and metabolite/parent drug ratio (N-CLB/CLB), suggestive of a putative poor metabolizer (PM) phenotype, were tested for CYP2CI9 polymorphisms. Eleven epileptic patients on stable CLB therapy were included for reference values of CLB and N-CLB metabolic variables and tested for possible CYP2CI9 polymorphisms. Detection of the CYP2C19*2, CYP2C19*3, and CYP2C19*4 mutations was performed in the genomic DNA by PCR amplification and enzyme digestion procedures. In the two presumed CYP2C19 PM patients, the N-CLB/CLB ratio was 10- to 27-fold higher than matched median values of the control epileptic patients. According to CYP2C19 genotyping, one patient was homozygous for CYP2C19*2, while the second presented only one copy of the same mutation, a genotype also found in three control patients. These observations provide further indirect in vivo evidence of CYP2CI9 isoenzyme involvement in the metabolism of the CLB main metabolite. According to genotyping, subjects carrying one or two copies of the defective CYP2C19*2 allele might develop markedly elevated steady-state plasma concentrations of N-CLB and be at higher risk of adverse effects.