The interrelationship between the etiology of pleural effusion and the side of its occurrence has been a subject of clinical interest for many years. It has often been stated that pleural effusion caused by congestive heart failure tends to occur on the right side. However some authors contended that such a side proclivity is not reliable. In the present study the suthors investigated the laterality of effusion caused by varous car-diovascular diseases complicated by heart failure. We reviewed the chest x-ray films and clinical records of 68 patients with proven pleural effusion resulted from heart failure. 1. There were 34 men and 34 women with the age ranging from 10 to 84 years with the mean of 59. 2. The diagnoses were rheumatic heart disease (2 patients) coronary heart disease(10 patients) hypertensive heart disease (8 patients) mitral valvular disease(18 patients) aortic valvular disease(3 patients) and miscellaneous(27 patients). 3. 34 patients had unilateral pleural effusion 25 and 9 right and left respectively. Of the 34 patients with bilaternal effusion 13 patients had predominently right-sided effusion 5 patients had predominently left-sided effusion and 16 patients had evenly distributed bilateral effusion. Thus, 55.8% of effusion was right-sided.s