This study sought to validate the accuracy of magnetic electroanatomic mapping (MEAM) for determining cardiac chamber size in a clinically relevant situation. The authors chose to compare LA size measured by MEAM to that assessed by two-dimensional guided M-mode echocardiography. The study included 37 patients with drug refractory paroxysmal atrial fibrillation who underwent two-dimensional echocardiography and a detailed MEAM of the LA. The entire LA was mapped with a mean of 132 +/- 50 points with attention to identifying the mitral annulus and posterior wall of the LA. The MEAM measurement of LA size was taken as the distance from the anterior wall of the LA to the posterior wall in a plane parallel to the mitral valve annulus at atrial end-diastole. LA dimension determined by M-mode echocardiography was assessed in a plane parallel to the mitral valve annulus in the parasternal long-axis view during atrial end-diastole. LA size assessed by M-mode echocardiography was 41.2 +/- 5.0 versus 40.9 +/- 4.5 mm as assessed by MEAM, with good correlation (r = 0.87, P < 0.001). Only three patients had a difference in LA size that was > 0.3 cm between the two measurement techniques. Thus, it appears that LA anteroposterior dimension as determined by electroanatomic mapping is similar to that determined by two-dimensional guided M-mode echocardiography. MEAM appears to be an accurate method by which LA size can be assessed in patients with drug refractory atrial fibrillation undergoing left atrial ablation procedures.