Atrial fibrillation and left atrial enlargement: cause or effect? In a blinded controlled study, 58 consecutive patients with definite left atrial enlargement (M-mode dimension of at least 45 mm) were followed up after 1-2 years. The aim of the study was to examine the following: (a) the prospective risk of developing atrial fibrillation (AF); and (b) the effect of the heart rhythm on the left atrial size. Of 36 patients in sinus rhythm, one developed paroxysmal AF and one developed persistent AF during a median follow-up period of 20 months. Thus the incidence of new AF was 5% per year. Eighteen patients died before scheduled echocardiographic follow-up, but in the remaining subjects the left atrial dimension did not change significantly: the median increment was 1 mm in 20 patients who sustained sinus rhythm vs 2 mm in 16 patients with chronic AF (P greater than 0.05). Although left atrial dilatation may cause AF and vice versa, this study demonstrated that the incidence of new AF is low, despite the fact that the left atrial dimension is substantially increased. Similarly, AF per se does not appear to have any major impact on the left atrial dimension. 