Background: Abnormal prolongation of QRS duration is a common finding in patients with chronic heart failure, and is associated with an impaired prognosis. The optimum QRS duration for separating chronic heart failure patients with respect to prognosis has not been determined. Whilst resynchronisation of ventricular conduction may benefit patients with QRS>150 ms, this has yet to be determined for patients with moderate QRS prolongation. Methods: We evaluated 155 patients with chronic heart failure (New York Heart Association class 2.6±0.8, mean±S.D.). The mean follow-up period was 838±748 days. Patients were sub-grouped according to QRS duration: <120 ms (normal QRS, n=82), 120–150 ms (moderate prolongation, n=44) and >150 ms (severe prolongation, n=29). Results: The optimal QRS duration for stratifying patients for 2-year event free survival was 120 ms (receiver operating characteristic analysis: area under curve 0.73; 95% CI 0.64–0.81). Moderate prolongation of QRS duration was associated with a worse New York Heart Association class, peak oxygen consumption and left ventricular ejection fraction when compared to patients with normal QRS duration (all P<0.05). Patients with moderate prolongation of QRS duration had similar impairment of New York Heart Association class and peak oxygen consumption as compared with patients with QRS duration >150 ms (all P>0.05). Conclusions: The optimum QRS duration for stratifying patients for medium to long-term event-free survival was 120 ms. Heart failure patients with moderate QRS prolongation share similar impairment of exercise capacity and functional class to those with severe prolongation.