AIM: This study compares different parameters of right atrial size evaluated by two-dimensional (2D) echocardiography with right atrial volume measured using three-dimensional echocardiography (3DE). METHODS AND RESULTS: One hundred sixty-three consecutive patients with a history of atrial arrhythmias were studied by standard two-dimensional and by real time 3DE. Of these 142 (87%) recordings were of sufficient quality for interpretation of the right atrium by both imaging techniques. The following parameters of right atrial size were measured: apical four-chamber short-axis diameter (4CH short axis), apical four-chamber long axis diameter (4CH long axis), and apical four-chamber planimetry area. The 2D-derived right atrial volume was calculated by using the single plane area-length method (4CH area-length). The 2D parameters were then correlated with right atrial volume measured by real time 3DE. Linear regression analysis showed moderate correlation for four-chamber planimetry area (r = 0.72, P < 0.001) and 2D-derived volume calculation (r of 4CH single plane area-length RA volume = 0.70, P < 0.001). Diameters correlated clearly less well with 3DE volume (r of 4CH short axis = 0.61, 4CH long axis = 0.59, P < 0.001 respectively). CONCLUSION: Real time 3DE is highly feasible for right atrial volume determination. The results demonstrate that measurements of dimensions using 2D echocardiography may not accurately assess right atrial size. If 3DE is not available, apical 4CH planimetry area is a simple alternative that may be used for evaluating right atrial size in clinical practice. The 2D-derived right atrial volume by single plane area-length method was not better correlated with 3DE volume than four-chamber planimetry area.