Cibenzoline, a Class I antiarrhythmic agent, can attenuate the left ventricular pressure gradient (LVPG) in patients with hypertrophic obstructive cardiomyopathy (HOCM). An association between the insertion/deletion polymorphism of the angiotensin-converting enzyme (ACE) gene and the progression of left ventricular hypertrophy in patients with hypertrophic cardiomyopathy has been reported. The aim of this study was to investigate the pharmacogenetic interactions between the ACE insertion/deletion polymorphism and the response to cibenzoline in patients with HOCM. Twenty-four patients with HOCM participated in this study. The LVPG and left ventricular function were measured by echocardiography before and 2 hours after administration of a single oral dose of 200 mg cibenzoline. The ACE insertion/deletion polymorphism was genotyped. The frequencies of the genotypes D/D, D/I, and I/I were 16%, 42%, and 42%, respectively. Before administration of cibenzoline, the LVPG was higher in patients with the D allele than in those without it (105 +/- 47 mm Hg versus 64 +/- 24 mm Hg, P = 0.0195). After administration of cibenzoline, the LVPG significantly decreased to 41 +/- 27 mm Hg in those with the D allele (P = 0.0001) and to 33 +/- 24 mm Hg in those without it (P = 0.0003). The LVPG in patients with the D allele was significantly decreased by cibenzoline when compared with patients without the allele (64 +/- 45 mm Hg versus 31 +/- 17 mm Hg, P = 0.038). Patients with HOCM with the ACE D allele had a high LVPG. Cibenzoline was more effective in patients with HOCM with the ACE D allele.