Intracavitary tumors of the atria are rare, generally diagnosed as myxoma, and considered benign. Of 20 such tumors presenting for surgery between Jan 1, 1967, and Jan 1, 1982, sixteen were myxomas: 13 in the left and three in the right atrium. Four additional patients operated on for suspected right atrial myxoma had other lesions: two thrombus (simulating myxoma), one leiomyoma originating in the right iliac vein, and one clear cell adenocarcinoma from the left kidney. One initially benign myxoma underwent malignant transformation. Eighteen hospital survivors were followed up from one month to 134 months (mean, 61 months). All are New York Heart Association class I. We recommend inferior vena cavagram for right-sided lesions. For patients older than 40 years, coronary angiography and appropriate bypass grafting should be performed. Surgery must include resection of all abnormal tissue. Follow-up with echocardiography should continue for five years. © 1983, American Medical Association. All rights reserved.