Patients can present with hypertrophied papillary muscles in the left ventricle, even without hypertrophy in other segments, and they have electrocardiographic (ECG) abnormalities suggestive of hypertrophic cardiomyopathy (HCM). This study was performed to evaluate whether the solitary papillary muscle hypertrophy was related to HCM. By analyzing 6731 echocardiographic studies between 1990 and 1994, the incidence of patients with papillary muscle hypertrophy was retrospectively examined, as well as the ECG features and family history related to HCM in these patients. After the normal size of the anterolateral and posteromedial papillary muscles was obtained from echocardiographic studies in 40 healthy subjects (0.7±0.2 cm for each of the vertical and horizontal axis), papillary muscle hypertrophy was defined as follows: either the vertical or horizontal diameter of at least one of the 2 papillary muscles was more than 1.1 cm (mean + 2SD in the normal subjects). Using this definition, 29 patients with papillary muscle hypertrophy were identified, of whom 14 (48%) showed high voltage QRS complexes, 10 (34%) showed T wave inversion, and 6 (21%) showed abnormal Q waves. Ten patients (34%) had a family history of HCM. In 2 patients that were followed for 18 and 11 years, respectively, the voltages of the QRS complexes and inverted T waves progressed with the hypertrophy of the papillary muscle. These findings suggest that solitary papillary muscle hypertrophy is related to HCM and that papillary muscle hypertrophy is a newly identified subtype of or an early form of HCM. (Jpn Circ J 1998; 62: 811 - 816)