Background: Although carvedilol and bisoprolol are effective medicines for the treatment of patients with heart failure (HF), only a few reports have compared their effects. This study was designed to compare the effects of them in patients with severe HF. Methods and Results: A total of 655 consecutive patients with HF, categorized as New York Heart Association Class 3 or 4, were retrospectively investigated. Of these patients, 217 were administered β-blockers after admission and were divided into 2 groups (carvedilol, n=110; bisoprolol, n=107). No significant differences were observed in their characteristics between the 2 groups prior to the introduction of the β-blockers. After 18 months of follow-up, there were no significant differences in the survival and cardiac event-free rates between the 2 groups. In contrast, there were several significant differences in patients with atrial fibrillation (AF) (carvedilol, n=40; bisoprolol, n=43). The percent changes in heart rate and brain natriuretic peptide level improved significantly in the bisoprolol group than in the carvedilol group. Furthermore, more patients in the bisoprolol group were defibrillated from AF to sinus rhythm than those in the carvedilol group (48% vs 16%; P=0.03). Conclusions: Our data suggest that the 2 β-blockers are equally effective in the improvement of severe HF, but bisoprolol shows favorable effects in patients with AF.  (Circ J 2010; 74: 1127 - 1134)