Recently, a novel allelic variant of cytochrome P450 2C19 encoding ultrarapid enzyme activity was described (denoted CYP2C1917). The objective of this study was to evaluate the impact of CYP2C1917 on serum concentration of escitalopram in psychiatric patients. One hundred and sixty-six patients treated with escitalopram were divided into the following subgroups according to CYP2C19 genotype: CYP2C1917/17 (n=7), CYP2C191/17 (n=43), CYP2C191/1 (n=60), CYP2C1917/def (n=16), CYP2C191/def (n=34), and CYP2C19def/def (n=6) (def=defective allele, i.e., CYP2C192 or 3). Dose-adjusted serum concentrations of escitalopram were compared using the CYP2C191/1 subgroup as reference. Geometric mean of the escitalopram serum concentration was 42% lower in patients homozygous for CYP2C1917 (P<0.01) and 5.7-fold higher in subjects homozygous for defective CYP2C19 alleles (P<0.001). Of the heterozygous subgroups, only CYP2C191/def was significantly different from CYP2C19*1/1 (P<0.001). In conclusion, a homozygous CYP2C1917 genotype is associated with lower serum concentration of escitalopram, which might imply increased risk of therapeutic failure.