Introduction: Current guidelines recommend cardiac resynchronisation therapy (CRT) in patients with severe symptomatic heart failure, depressed left ventricular (LV) systolic function and a wide QRS complex (≥120 ms). However, patients with heart failure having a narrow QRS complex might also benefit from CRT. Design setting patients interventions: During the Predictors of Response to Cardiac Resynchronisation Therapy (PROSPECT) trial, 41 patients were enrolled in a 'narrow' QRS sub-study. These patients had a QRS complex <130 ms, but documented evidence of mechanical dyssynchrony by any of seven pre-defined echocardiographic measures. Results: After 6 months of CRT, 26 (63.4%) patients showed improvement according to the Clinical Composite Score, 4 (9.8%) remained unchanged and 11 (26.8%) worsened. In patients with paired data, the 6-min walking distance increased from 334±118 m to 382±128 m, (p=0.003) and quality-of-life score improved from 44.2±19.7 to 26.8±20.2 (p<0.0001). Furthermore, there was a significant decrease in LV end-systolic diameter (from 59±9 to 55±12 mm, p=0.002) and in LV end-diastolic diameter (from 67±9 to 63±11 mm, p=0.007). Conclusion: The results suggest that CRT may have a beneficial effect in heart failure patients with a narrow QRS complex and mechanical dyssynchrony as assessed by echocardiography. The majority of patients improved on clinical symptoms, and there was an evident reduction in LV diameters. Larger studies are needed to clearly define selection criteria for CRT in patients with a narrow QRS complex.