Relatively little attention has been paid to the frequency of atrial fibrillation (AF) in patients with mitral regurgitation (MR) secondary to mitral valve prolapse (MVP). We reviewed clinical, electrocardiographic, echocardiographic, hemodynamic, and angiographic findings in 246 patients aged 21 to 84 years (mean 61) (66% men) who had mitral valve repair or replacement for MR secondary to MVP. Immediately before the mitral operation by electrocardiogram, only 37 patients (15%) had AF and the other 209 patients were in sinus rhythm. Of the latter, 32 had had a history of AF that had reverted to sinus rhythm spontaneously or with antiarrhythmic therapy. Thus, a total of 69 patients (28%) had AF at some time. In conclusion, the frequency of AF in patients with MR secondary to MVP and sick enough to warrant a mitral valve operation have a relatively low frequency of AF (persistent in 15%, paroxysmal in another 13%), percentages considerably lower than that seen in patients with mitral stenosis just before a mitral commissurotomy or replacement.