OBJECTIVE--To assess the clinical characteristics of patients in whom cardiac function improved after cardioversion of atrial fibrillation and the time course of the improvement. DESIGN--A prospective serial study of echocardiograms recorded before cardioversion and one day, seven days, one month, and three months after cardioversion. SETTING--Echocardiography laboratory of a university hospital. PATIENTS--23 patients with chronic atrial fibrillation in whom cardioversion was successful. MAIN OUTCOME MEASURES--M mode indices of the left ventricular wall motion and pulsed Doppler indices of the left ventricular inflow. RESULTS--Three months after cardioversion percentage fractional shortening had increased by more than 5% in 14 patients (improved group) and by less than 5% in nine patients (non-improved group). Those in whom cardiac function improved had significantly higher heart rates and a greater reduction in ventricular filling during atrial fibrillation and a more prominent atrial filling wave three months after cardioversion than those patients in the non-improved group. Over the three months of follow up the mean (1SD) percentage fractional shortening increased from 22 (3)% to 30 (4)% in the improved group and in this group heart rate fell one day after cardioversion. A month after cardioversion the percentage fractional shortening had increased to 35 (5)% and the atrial systolic contribution to left ventricular filling increased from 30 (9)% on day 1 to 47 (12)%. CONCLUSIONS--Cardioversion improved cardiac function in patients with tachycardia and reduced ventricular filling during atrial fibrillation. Because both an immediate reduction of heart rate and a delayed recovery of atrial booster pump function played an important part in the improvement of cardiac function the long-term effects of cardioversion should be assessed at least a month after cardioversion.