Background Abnormal papillary muscles(PM) are often found in hypertrophic cardiomyopathy(HCM) and the significance relating left ventricular(LV) outflow obstruction has been identified. Mid-ventricular obstruction(MVO) can be found in apical HCM and accompanied by apical aneurysm and ventricular tachycardia. We sought to investigate PM structures including numbers, thickness, insertion site and their association with MVO in apical HCM. Methods In 44 patients(30 men, age 57짹8 yrs) with apical HCM, cardiac magnetic resonsnce was performed. PM numbers, maximal PM thickness in diastole, and their insertion site on LV were evaluated. Patients with isolated asymmetric apical hypertrophy were classified as a pure type, and those with coexistent hypertrophy of the interventricular septum as a mixed type. MVO were examined using Doppler echocardiography and diagnosed when the systolic peak flow velocity at the mid-ventricle was���2.5 m/s. Subjects were divided into 2 groups according to the presence of MVO; Group I(n=34): MVO(-), Group II(n=10): MVO(+). Results The mixed type was more prevalent(54 vs 93%, p=0.014) and posterior wall thickness at mid LV was higher(10.2짹1.8 vs 12.9짹2.1, p<0.001) in group II compared with group I. Apical aneurysm was found in 3 patients in group II. Group II showed higher PM numbers(2.7짹0.8 vs 3.3짹0.8, p=0.070) and maximal thickness(8.7짹0.9 vs 10.8짹1.5, p=0.045). PMs were displaced closer to the LV apex(displaced apically: 68 vs 93%, p=0.042) in almost of group II patients. There was no significant difference in heart rate between two groups. ConclusionIn apical HCM, patients with MVO showed different PM structures and displacement than those without MVO. Increased number of PMs and their apical displacement can be one of contributors of MVO. Combined with the LV morphologic type, the assessement of PM morphology would be important to predict the development of MVO and its subsequent outcomes in apical HCM.