It has been generally assumed that most patients with hypertrophic cardiomyopathy (HC) who develop atrial fibrillation (AF) have marked left ventricular (LV) hypertrophy and subaortic obstruction. The morphologic and functional features of this subset of patients with HC have not been systematically investigated. The LV morphology and functional profile of 46 patients with HC and chronic AF were compared with those of 81 control patients with HC and normal sinus rhythm. Contrary to expectations, LV hypertrophy (assessed with 2-dimensional echocardiography) was substantially less marked in the patients with AF than in the control patients, and prevalence of subaortic obstruction was similar in the 2 groups. Maximal LV wall thickness and wall thickness index were lower in patients with AF (18 +/- 2 and 56 +/- 7 mm, respectively) than in control patients (22 +/- 6 and 67 +/- 16 mm, respectively; p less than 0.001). Furthermore, mild LV hypertrophy (maximal LV wall thickness less than or equal to 17 mm confined to 1 ventricular segment) was almost twice as frequent in patients with AF (63%) than in control patients (36%; p less than 0.005). Subaortic obstruction was present in 9 patients with AF (20%) and in 28 control patients (35%; p greater than 0.05). In a subgroup of 22 patients with AF who were followed for 4 to 10 years, 5 patients had marked LV wall thinning (greater than or equal to 5 mm, range 5 to 14). In conclusion, these results demonstrate that most patients with HC and chronic AF have the nonobstructive form of HC, and relatively mild LV hypertrophy.