The normal motion of a left ventricular catheter parallels that of the aortic root; it moves anterior during systole and posterior during diastole. In contrast, a prolapsing left atrial myxoma causes paradoxical motion of the catheter; posterior during systole and anterior during diastole. Paradoxical motion was found in each of five cases of prolapsing left atrial myxoma (no false negatives), and in six out of 61 controls (six false positives). In the false positive cases, the catheter was not positioned on the ventricular floor and usually only minor degrees of abnormal motion were present. Paradoxical motion of the left ventricular catheter is an ancillary angiographic finding in prolapsing left atrial tumor.