To explore the pharmacogenetic effects of the cytochrome P450 (CYP) 2D6 genotype in patients with systolic heart failure treated using controlled/extended-release (CR/XL) metoprolol, this study assessed the CYP2D6 locus for the nonfunctional * 4 allele (1846G> A; rs3892097) in the Metoprolol CR/XL Randomised Intervention Trial in Congestive Heart Failure (MERIT-HF; n = 605). Participants were characterized as extensive, intermediate, or poor metabolizers (EMs, IMs, or PMs, respectively), based on the presence of the CYP2D6* 4 allele (EM: * 1* 1, 60.4%; IM: * 1* 4, 35.8%; and PM: * 4* 4, 3.8%). Plasma metoprolol concentrations were 2.1-/4.6-fold greater in the IM/PM groups as compared with the EM group (P <0.0001). Metoprolol induced significantly lower heart rates and diastolic blood pressures during early titration, indicating a CYP2D6* 4 allele dose-response effect (P <0.05). These effects were not observed at maximal dose, suggesting a saturable effect. Genotype did not adversely affect surrogate treatment efficacy. CYP2D6 genotype modulates metoprolol pharmacokinetics/pharmacodynamics during early titration; however, the MERIT-HF-defined titration schedule remains recommended for all patients, regardless of genotype.