A patient with a left atrial myxoma is reported in whom M-mode echocardiography was atypical and misleading since in most transducer positions, no tumor echoes were found in the left atrium. Real time two-dimensional echocardiography provided a definitive diagnosis, demonstrating a large left atrial mass with multiple areas of calcification. One such calcified area was located adjacent to the posterior aortic wall, and the tumor echoes were sparsely imaged directly behind it due to incomplete penetration of the ultrasound beam through dense calcification (acoustic shadowing). Difficulty in M-mode diagnosis appeared to result from this acoustic shadowing effect. In our patient, two-dimensional echocardiography was superior to the M-mode in making a definitive diagnosis of left atrial tumor because of the difficulty visualizing tumor echoes in the left atrium.