Clinical study was done on 119 patients with atrial fibrillation in the aspect of underlying diseases, particulary. The results were as follow : 1) In the underlying diseases of atrial finbrillation, rheumatic heart disease, hepertension and ischemic heart disease were common, which occupied 27.8%, 22.7% and and 15.1%, respectly. No apparent underlying disease was found in 21.0%, but in the transient atrial fibrillation, no apparent underlying disease in 42.9%. 2) There were no significant correlations between the ventricular rate, amplitude of f wave and underlying diseases of atrial fibrillation. 3) Complications were found in 29.4% of all cases, such as congestive heart failure(22.7%), embolism(6.7%). Otherwise, atrial fibrillation without underying cardiovascular diseases, congestive heart failure was found only(7.7%).