AIMS: To assess the cost-effectiveness of cardiac resynchronization therapy (CRT) compared with optimal medical therapy in patients with New York Heart Association (NYHA) II heart failure (HF) or NYHA I with previous HF symptoms. METHODS AND RESULTS: A proportion in state model with Monte Carlo simulation was developed to assess the costs, life years and quality-adjusted life year (QALYs) associated with CRT-ON and -OFF over a 10 year time period. Data from 262 patients in the European cohort of the REVERSE clinical trial (QRS ≥ 120 ms, left ventricular ejection fraction ≤ 40%, CRT-ON, n = 180, CRT-OFF, n = 82) were used to model all-cause mortality, change in NYHA class and resource use. EQ-5D preference weights were taken from a previous cost-effectiveness model of CRT and unit costs from national UK databases. Costs and benefits were discounted at 3.5% p.a. Extensive deterministic and probabilistic sensitivity analyses were performed. Compared with CRT-OFF, 0.94 life years or 0.80 QALYs were gained in the CRT ON group at an additional cost of €11 455, yielding an incremental cost-effectiveness ratio of €14.278 per quality-adjusted life year (QALY) gained. At a threshold of €33 000 (£30 000) per QALY gained, the probability that CRT is cost-effective is 79.6%. Cardiac resynchronization therapy becomes cost effective after ∼4.5 years. Cardiac resynchronization therapy needs only to demonstrate a modest impact on all cause mortality (hazard ratio = 0.82) in order to demonstrate cost-effectiveness. The results are robust to changes in all other parameters. CONCLUSION: Cardiac resynchronization therapy is a cost-effective intervention for patients with mildly symptomatic HF and for asymptomatic patients with left ventricular dysfunction and previous HF symptoms.