Biventricular resynchronization has been shown to be beneficial on morbidity and mortality in patients with symptomatic (NYHA class III or IV) systolic heart failure (left ventricular ejection fraction or LVEF is less or equal to 35%) under optimal medical treatment with electrical asynchrony (QRS > or = 120 ms) and in sinus rhythm. The purpose of this study was to evaluate the efficacy and safety of upgrading to biventricular resynchronization in paced patients presenting with symptomatic systolic heart failure. Over a period of eight years, 24 paced patients with symptomatic (class III or IV) systolic heart failure (LVEF < or =35%) with electrical asynchrony (QRS > or =160 ms) received an additional left ventricular pacing lead and a biventricular pulse generator. We compared the functional symptoms, QRS duration, LVEF, left ventricle telediastolic diameter and any aggravation or onset of ventricular arrhythmia before and after biventricular resynchronization. Biventricular resynchronization led to an improvement in dyspnea in 80% of cases (one or more class decrease on NHYA scale), a significant shortening in QRS duration (-40 ms, p < 0.05), a significant improvement in left ventricular dilation (-4 mm, p < 0.05) and a significant improvement in the ejection fraction (+4%, p < 0.05). This study showed that in paced patients presenting with cardiac failure and systolic dysfunction refractory to medical treatment, upgrading from a conventional pacing system to a biventricular pacemaker leads to a significant improvement in functional symptoms.