To elucidate the clinical features of hypertrophic cardiomyopathy (HCM) with atrial fibrillation (Af), the complications and prognosis of 18 patients with HCM complicated by Af (Af group) were compared with those of 20 patients without Af (non-Af group). The Af group was categorized depending on duration as 1. Af group I (four patients) with persistent Af at diagnosis, 2. Af group II (10 patients) with transient Af of more than five hours' duration, and 3. Af group III (four patients) with Af less than five hours' duration. The results were as follows: 1. During the follow-up period, among the 18 patients in the Af group, five including four patients in the Af group I, developed arterial embolism, 10 developed congestive heart failure, and three died suddenly. The patients in the non-Af group had excellent courses without serious complications. 2. Electrical cardioversion was performed in six patients in the Af group II, and sinus rhythm was restored in all of them, resulting in improved clinical courses. Thus, no complications developed in four patients whose sinus rhythm was maintained and whose clinical courses were favorable. 3. The P-terminal force in V1 prior to onset of Af in the Af groups II and III, left atrial dimension, and the cardiothoracic ratio in the Af group, were significantly greater than that of the non-Af group (p less than 0.01, p less than 0.05, p less than 0.05, respectively). In conclusion, HCM associated with Af has a poor prognosis, with severe complications such as heart failure, arterial embolism and sudden death. Cardioversion may be an effective method of choice of intervention in patients with recently-developed Af. P-terminal force in V1, left atrial dimension, and cardiothoracic ratio are regarded important factors in the development of Af.