To evaluate the cost-effectiveness of combined resynchronisation and implantable defibrillator therapy for left ventricular dysfunction and explore subgroups in which such devices might be most cost-effective.Markov model-based economic evaluation.UK NHS.A simulated mixed age cohort of NYHA class III and IV patients with left ventricular systolic dysfunction and prolonged QRS interval.Cost per quality adjusted life year gained over the patient lifetime.The incremental cost-effectiveness of resynchronisation therapy alone compared with optimal medical therapy was £16,735 (95% CI: £14,630 to £20,333) with a 91% probability of being cost-effective at a willingness to pay threshold of £30,000. Compared with resynchronisation alone, the incremental cost-effectiveness of combined implantable defibrillator was £40,160 (95% CI: £26,645 to £59,391) with only a 26% probability of cost-effectiveness at the £30,000 threshold. In a direct comparison across three treatments (medical treatment, resynchronisation alone and combined resynchronisation with implantable defibrillator therapy) resynchronisation alone was found to be the most cost-effective option.Combined resynchronisation and implantable defibrillator therapy is not cost-effective for left ventricular dysfunction. Instead resynchronisation alone remains the most cost-effective policy option in this population. Combined devices are more likely to be cost-effective in the subgroups of younger patients or those with high risk of sudden cardiac death who would qualify for resynchronisation therapy.