The coincidence of mutated alleles of CYP2C18 and CYP2C19 was studied in 154 Japanese subjects. The mutant alleles of CYP2C18 studied were CYP2C18m1 (T204 --> A substitution in exon 2) and CYP2C18mFR (A-460 --> T substitution in the 5'-flanking region), and those of CYP2C19 were CYP2C19m1 (G689 --> A substitution in exon 5) and CYP2C19m2 (G636 --> A substitution in exon 4). They were identified by polymerase chain reaction and restriction fragment length polymorphism. The results indicate that genotypes of CYP2C18m1 and CYP2C18mFR are completely coincident with those of CYP2C19m2 and CYP2C19m1, respectively. The finding suggests that the mutations of CYP2C18 and CYP2C19 examined in the present study are very closely linked with each other (i.e. CYP2C18m1 vs CYP2C19m2 and CYP2C18mFR vs CYP2C19m1), at least in a Japanese population.