Restoration of sinus rhythm may improve functional capacity in atrial fibrillation in the short-term. Little is known, however, about its long-term effect on functional status. The aim of the present study was to evaluate the long-term effect of cardioversion on peak oxygen consumption (VO2) in patients with chronic atrial fibrillation. Patients with such a condition and due to undergo electrical cardioversion were eligible for the study. Patients underwent treadmill exercise testing with measurement of peak VO2 before cardioversion, and at 1 month and 2 years thereafter. Based on the rhythm present at those times after cardioversion, patients were categorized into three groups: those in sinus rhythm after 1 month and 2 years (Group I); those in sinus rhythm after 1 month, but with atrial fibrillation after 2 years (Group II); and those who were in atrial fibrillation both at 1 month and 2 years following cardioversion (Group III). Thirty-nine patients were included, and underlying heart disease was present in 24 of them (62%). In the comparison of the baseline characteristics of Group I (n = 17), Group II (n = 11), and Group III (n = 11), underlying heart disease was more frequent in Group I (88%, 45%, and 36%, respectively); otherwise they were similar. In Group I, peak VO2 showed an insignificant increase from 21.1 +/- 5.0 to 22.3 +/- 5.0 ml.min-1.kg-1 month after. cardioversion. After 2 years of sinus rhythm, peak VO2 showed a further increase to 23.8 +/- 5.0 ml.min-1.kg-1 (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)