Up to 50% of patients with symptoms of heart failure have a preserved left ventricular ejection fraction (HFpEF syndrome). Current therapy for systolic heart failure is targeted at inducing vasodilation and counteracting neuro-endocrine activation. There is a lack of a corresponding evidence base for the treatment of HEpEF. We have previously shown that perhexiline, an agent that increases the efficiency of energy production by shifting substrate utilization from free fatty acids towards glucose, was highly effective in improving exercise capacity, symptoms and cardiac function in patients with systolic heart failure. We have also recently shown that energy deficiency plays a major role in the pathophysiology of HFpEF. In this proposal we therefore aim to investigate the effectiveness of perhexiline in 70 HFpEF patients, in a 3 month randomised, double-blind, controlled trial. An interim analysis is planned after 20 patients.